(CNN) U.S. Secretary of State John Kerry had strong words Saturday for Russia over what he called its unacceptable actions in Ukraine -- and threatened additional sanctions against Russian interests.

Speaking in London alongside UK Foreign Secretary Philip Hammond, Kerry said Russia was "continuing to do land-grabbing in Ukraine" even as it professed to support peace efforts.

Russia's actions are "just simply unacceptable, so we are talking about additional sanctions, additional efforts," Kerry said.

A Ukrainian volunteer fighter fires a machine gun at pro-Russian rebels near the village of Pisky, Ukraine, on Saturday, January 3.

A Ukrainian volunteer fighter fires a machine gun at pro-Russian rebels near the village of Pisky, Ukraine, on Saturday, January 3.

Ukrainian President Petro Poroshenko gives a speech as he hands over new military equipment to forces near the city of Ghytomyr, Ukraine, on Monday, January 5.

Ukrainian President Petro Poroshenko gives a speech as he hands over new military equipment to forces near the city of Ghytomyr, Ukraine, on Monday, January 5.

A Ukrainian soldier looks down from a military truck at the Donetsk airport on Tuesday, January 6. The airport has been the scene of some of the fiercest fighting in eastern Ukraine.

A Ukrainian soldier looks down from a military truck at the Donetsk airport on Tuesday, January 6. The airport has been the scene of some of the fiercest fighting in eastern Ukraine.

Men from the Azov Volunteer Battalion board a bus in Kiev to join the fight against the rebels on Saturday, January 17.

Men from the Azov Volunteer Battalion board a bus in Kiev to join the fight against the rebels on Saturday, January 17.

A building hit by Ukrainian artillery is seen in the Voroshilovsky area of Donetsk on January 18.

A building hit by Ukrainian artillery is seen in the Voroshilovsky area of Donetsk on January 18.

Women sit in a shelter during a battle in Donetsk on Sunday, January 18.

Women sit in a shelter during a battle in Donetsk on Sunday, January 18.

Vladimir Bovrichev cries next to the body of his 4-year-old son, Artiam, during Artiam's funeral on the outskirts of Donetsk on Tuesday, January 20. The boy was killed during a Ukrainian artillery strike.

Vladimir Bovrichev cries next to the body of his 4-year-old son, Artiam, during Artiam's funeral on the outskirts of Donetsk on Tuesday, January 20. The boy was killed during a Ukrainian artillery strike.

Rubble and debris cover the airport in Donetsk on Wednesday, January 21.

Rubble and debris cover the airport in Donetsk on Wednesday, January 21.

A rebel takes aim while protecting a supply position in the Kievsky district of Donetsk on January 22.

A rebel takes aim while protecting a supply position in the Kievsky district of Donetsk on January 22.

A trolleybus is damaged in Donetsk's Lenin District after its station was hit by a shell on January 22.

A trolleybus is damaged in Donetsk's Lenin District after its station was hit by a shell on January 22.

People in downtown Donetsk react as Ukrainian prisoners of war are handed over by pro-Russian rebels on January 22.

People in downtown Donetsk react as Ukrainian prisoners of war are handed over by pro-Russian rebels on January 22.

A pro-Russian rebel takes cover from shelling in the Kievsky district of Donetsk on Thursday, January 22.

A pro-Russian rebel takes cover from shelling in the Kievsky district of Donetsk on Thursday, January 22.

A piece of an exploded missile sits lodged in the ground outside an apartment building in the Vostochniy district of Mariupol on Sunday, January 25.

A piece of an exploded missile sits lodged in the ground outside an apartment building in the Vostochniy district of Mariupol on Sunday, January 25.

A man injured during shelling in Mariupol sits in an emergency hospital on January 26.

A man injured during shelling in Mariupol sits in an emergency hospital on January 26.

Ukrainian servicemen prepare ammunition at a position on the front line near Mariupol on January 26.

Ukrainian servicemen prepare ammunition at a position on the front line near Mariupol on January 26.

People in Mariupol, Ukraine, pour soil into the grave of a recent shelling victim on Monday, January 26.

People in Mariupol, Ukraine, pour soil into the grave of a recent shelling victim on Monday, January 26.

The body of a civilian killed during shelling lies on the ground in Donetsk on Friday, January 30.

The body of a civilian killed during shelling lies on the ground in Donetsk on Friday, January 30.

A man stands next to his car in Donetsk on Sunday, February 1, after it was destroyed by shelling.

A man stands next to his car in Donetsk on Sunday, February 1, after it was destroyed by shelling.

A child waits on a bus to leave Debaltseve on Tuesday, February 3, after increased fighting in the region.

A child waits on a bus to leave Debaltseve on Tuesday, February 3, after increased fighting in the region.

Ukrainian volunteer fighters and policemen arrest two men in Kiev, Ukraine, on February 9. The men allegedly arrived from Donetsk and were suspected of participating in pro-Russian rebel activities and organizing terrorist attacks in the Ukrainian capital.

Ukrainian volunteer fighters and policemen arrest two men in Kiev, Ukraine, on February 9. The men allegedly arrived from Donetsk and were suspected of participating in pro-Russian rebel activities and organizing terrorist attacks in the Ukrainian capital.

A volunteer gets a medical checkup at a military base for pro-Russian rebels February 10 in Donetsk, Ukraine.

A volunteer gets a medical checkup at a military base for pro-Russian rebels February 10 in Donetsk, Ukraine.

People stand beside the body of a woman killed during shelling in Kramatorsk, Ukraine, on Tuesday, February 10.

People stand beside the body of a woman killed during shelling in Kramatorsk, Ukraine, on Tuesday, February 10.

A recent ceasefire was brokered during marathon talks in Minsk, Belarus. From left, Belarusian President Alexander Lukashenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, French President François Hollande and Ukrainian President Petro Poroshenko gather before negotiations begin on Wednesday, February 11.

A recent ceasefire was brokered during marathon talks in Minsk, Belarus. From left, Belarusian President Alexander Lukashenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, French President François Hollande and Ukrainian President Petro Poroshenko gather before negotiations begin on Wednesday, February 11.

People carry a refrigerator through a balcony at an apartment building that was damaged in recent shelling in Svitlodarsk on February 15.

People carry a refrigerator through a balcony at an apartment building that was damaged in recent shelling in Svitlodarsk on February 15.

A woman salvages items February 15 from the rubble of a destroyed clinic where she had worked in Opytne, Ukraine.

A woman salvages items February 15 from the rubble of a destroyed clinic where she had worked in Opytne, Ukraine.

Ukrainian servicemen play with a soccer ball on a road between Svitlodarsk and Debaltseve on February 15.

Ukrainian servicemen play with a soccer ball on a road between Svitlodarsk and Debaltseve on February 15.

An army ambulance damaged in recent shelling lies by a road near Svitlodarsk, Ukraine, on Sunday, February 15.

An army ambulance damaged in recent shelling lies by a road near Svitlodarsk, Ukraine, on Sunday, February 15.

A man holds a Ukrainian flag as he covers a victim of an explosion in Kharkiv, Ukraine, on Sunday, February 22. The explosion during a peaceful protest left two dead and 15 wounded.

A man holds a Ukrainian flag as he covers a victim of an explosion in Kharkiv, Ukraine, on Sunday, February 22. The explosion during a peaceful protest left two dead and 15 wounded.

A member of a Ukrainian military medical unit cries during a ceremony in Artemivsk, Ukraine, on February 23. Four of his comrades were killed near Debaltseve, Ukraine.

A member of a Ukrainian military medical unit cries during a ceremony in Artemivsk, Ukraine, on February 23. Four of his comrades were killed near Debaltseve, Ukraine.

A Ukrainian serviceman climbs out of a tank at a checkpoint near Horlivka, Ukraine, on Monday, February 23.

A Ukrainian serviceman climbs out of a tank at a checkpoint near Horlivka, Ukraine, on Monday, February 23.

A volunteer gives humanitarian aid to residents of Popasna, Ukraine, on Saturday, February 28.

A volunteer gives humanitarian aid to residents of Popasna, Ukraine, on Saturday, February 28.

A woman makes her way across a bridge destroyed in heavy fighting in Donetsk, Ukraine, on March 1.

A woman makes her way across a bridge destroyed in heavy fighting in Donetsk, Ukraine, on March 1.

An instructor of the Ukrainian volunteer Azov Battalion conducts training exercises in Kiev, Ukraine, on Sunday, March 1.

An instructor of the Ukrainian volunteer Azov Battalion conducts training exercises in Kiev, Ukraine, on Sunday, March 1.

A mortar round sticks out of the ground near a destroyed tank at a former Ukrainian army checkpoint outside Chornukhyne, Ukraine, on Monday, March 2. Russian-backed separatists had recently overran the area.

A mortar round sticks out of the ground near a destroyed tank at a former Ukrainian army checkpoint outside Chornukhyne, Ukraine, on Monday, March 2. Russian-backed separatists had recently overran the area.

Residents flee with salvaged belongings during renewed fighting in the Jabunki neighborhood near the airport in Donetsk on Monday, April 13.

Residents flee with salvaged belongings during renewed fighting in the Jabunki neighborhood near the airport in Donetsk on Monday, April 13.

An American soldier, right, trains Ukrainian troops on Tuesday, April 21, near Yavoriv, Ukraine. Operation Fearless Guardian, a six-month training exercise, involves about 300 members of the American 173rd Airborne and about 900 Ukrainian National Guard troops.

An American soldier, right, trains Ukrainian troops on Tuesday, April 21, near Yavoriv, Ukraine. Operation Fearless Guardian, a six-month training exercise, involves about 300 members of the American 173rd Airborne and about 900 Ukrainian National Guard troops.

Pro-Russian rebels carry the coffin of prominent separatist commander Alexei Mozgovoi during his funeral in Alchevsk, Ukraine, on Wednesday, May 27.

A Ukrainian serviceman fires a grenade launcher on the front lines near Donetsk on Saturday, May 30.

A Ukrainian serviceman fires a grenade launcher on the front lines near Donetsk on Saturday, May 30.

Shelling between Ukrainian troops and pro-Russian rebels leaves damage in Donetsk, Ukraine, on Monday, June 1.

Shelling between Ukrainian troops and pro-Russian rebels leaves damage in Donetsk, Ukraine, on Monday, June 1.

A man with a machine gun is among the Ukrainian troops standing guard in Krimskoe town of Luhansk, Ukraine, on June 25.

A man with a machine gun is among the Ukrainian troops standing guard in Krimskoe town of Luhansk, Ukraine, on June 25.

Ukrainian security forces on the lookout in Bobrovyshche on July 14. The country's troops face daily attacks from Russian-backed separatists despite a ceasefire being in place.

Ukrainian security forces on the lookout in Bobrovyshche on July 14. The country's troops face daily attacks from Russian-backed separatists despite a ceasefire being in place.

Ukrainian security forces patrol in the village of Bobrovyshche on July 14, 2015. More than 6,400 people have been killed in the conflict in Ukraine since April 2014, the United Nations says.

Ukrainian security forces patrol in the village of Bobrovyshche on July 14, 2015. More than 6,400 people have been killed in the conflict in Ukraine since April 2014, the United Nations says.

"We are confident that over the next few days we are going to make it clear that we are not going to play this game, not going to sit here and be part of this extraordinarily craven behavior at the expense of the sovereignty and integrity of a nation.

"This is behavior that is completely counter to everything that the global community has worked to achieve and put in place ever since World War II, and I'm confident that the United Kingdom, the United States and others are prepared to stand up to it."

He blasted Moscow's repeated denials that it is involved in arming the separatists and sending its own troops over the border.

In this age of technology and satellites, Kerry said, "there is no secret" over what is taking place. "We know to a certainty what Russia has been providing to the separatists, how Russia is involved with the separatists," he said.

Kiev and a number of Western leaders have steadfastly accused Russia of failing to rein in separatists and of continuing to arm, supply and train them. European nations, the United States and others have imposed financial sanctions on Russian interests in hopes of pressuring President Vladimir Putin to help end the hostilities.

Russia, which seized Ukraine's Crimean peninsula in March, denies any direct involvement in Ukraine and says any Russian soldiers in Ukraine are volunteers fighting during their vacation time

Debaltseve siege most deadly

Kerry's remarks come as a ceasefire which came into effect last Sunday remains in serious doubt.

A fierce separatist offensive to take the key strategic town of Debaltseve ended only Wednesday, when Ukrainian soldiers were forced to retreat.

Yuriy Biryukov, a presidential adviser and assistant to Ukraine's defense minister, said Saturday that 179 soldiers had died in the Debaltseve area between January 18 and February 18.

An additional 110 soldiers were captured and 81 are missing, Biryukov posted on Facebook. Some of the missing are still on their way out of Debaltseve, he said, while others have died.

Late Saturday, Ukrainian President Petro Poroshenko said 139 Ukrainian soldiers had been released from captivity.

The release was part of a prisoner swap between Ukrainian and rebel forces in the eastern Donbass region; 52 pro-Russian separatists were also freed, according to Russian state news agency Itar Tass.

The toll makes the siege of Debaltseve -- a town now shattered by heavy shelling -- the Ukrainian military's deadliest single incident over the past 10 turbulent months.

Shelling has also picked up this week in Donetsk city and clashes are reported around the southern port city of Mariupol.

Ukraine's National Defense and Security Council said Friday there had been 300 instances in which this latest truce has been violated.

The ceasefire agreement -- signed February 12 in Minsk, Belarus, among the leaders of Ukraine , Russia, France and Germany -- aimed to curb this violence. It is supposed to be followed by the withdrawal of heavy weaponry to create a buffer zone, the release of prisoners and steps toward new elections.

Months of devastating fighting has left nearly 5,700 people dead as of February 18, the U.N. Office of the High Commissioner for Human Rights reported Friday. More than 1 million people have been displaced.

Shot Maidan activists remembered

and in favor of closer ties to Europe. On Friday, Ukraine marked one year since the bloodiest day of protests in Kiev's Maidan, or Independence Square, against the country's then Russian-leaning leaderand in favor of closer ties to Europe.

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Some 49 people died on February 20, 2014, and close to 100 more suffered gunshot wounds when, according to protesters, government snipers opened fire on them.

Two days later, President Viktor Yanukovych fled the country, prompting Ukrainian activists to declare "victory in the Maidan" and promise a new day for a country long torn between its neighbors, Europe to the west and Russia to the east.

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How things have changed. Today, the country is a powder keg driving spiraling tensions between the West and Moscow.

In Moscow, thousands of Kremlin supporters gathered Saturday for a demonstration to counter the Ukrainian commemorations.

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Many waved flags and banners as they marched near Moscow's Red Square, while others chanted slogans including, "We won't give our country to the enemy" and "No Maidan in Russia."

In remarks Friday, Poroshenko claimed Russia had been working toward the breakup of Ukraine since before Yanukovych was ousted.

"Moscow was preparing to the liquidation and tearing Ukraine apart long before the victory of Maidan. They were expecting the fall of Yanukovych and accelerated the course of events," Poroshenko said to the families of protesters who died a year ago.

According to Ukrainian prosecutors, 77 people died in total during the 2014 protests, which were sparked by Yanukovych's decision to scrap a trade deal with the European Union and instead turn toward Russia.

By March, Russia had annexed Crimea. Weeks later, in April, pro-Russian separatist forces launched their bid to carve off the more Russian-leaning eastern Donetsk and Luhansk regions from the rest of Ukraine.

NATO deputy leader: Russia poses existential threat

In a speech to a London-based defense think-tank , the Royal United Services Institute, Britain's senior officer in NATO warned that Russia's expansionist ambitions could pose an "existential threat" to the world.

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Russia is currently employing a "hybrid combination" of "coercion through rapidly generated conventional forces and subversion, through a number of means, both military and non-military," said Gen. Adrian Bradshaw, NATO's Deputy Supreme Allied Commander Europe.

This hybrid strategy can be used to set the scene for a "subversive takeover of territory," as seen in its takeover of Ukraine's Crimean peninsula last year shortly after large-scale military exercises by Russia on its side of the border, he said.

"We are today seeing conventional forces employed, albeit subject to continued brazen denials by the Kremlin, in eastern Ukraine," he added.

Russia's new strategy holds particular dangers for NATO, he said. Firstly, the use of subversive tactics initially makes it difficult to identify clearly "the hand of a hostile state government" in destabilization of a country, making collective decisions harder.

Secondly, its ability rapidly to generate large scale forces "could in future be used not only for intimidation and coercion but potentially to seize NATO territory, after which the threat of escalation might be used to prevent reestablishment of territorial integrity," he said.

And, Bradshaw pointed out, Russia is not the only threat to NATO states, citing also the danger posed by terror groups such as ISIS, also known as ISIL.

"Whilst the threat from Russia, together with the risk it brings of a miscalculation resulting in a slide into strategic conflict, however unlikely we see that as being right now, represents an obvious existential threat to our whole being, we of course face threats from ISIL and other instabilities to our way of life and the security of our loved ones."