Washington (CNN) The serious escalation of fighting in Ukraine is pushing President Barack Obama closer to the proxy war with Russia that he has long sought to avoid.

That prospect is increasing as the United States openly accuses its former Cold War foe of sending tanks and troops in unmarked uniforms across the border into eastern Ukraine, boosting separatists in their battle against Kiev's beleaguered forces.

The revelations are sharpening Obama's dilemma as his White House considers whether to send "lethal" weapons, including anti-tank and anti-mortar systems to the outgunned Ukrainian armed forces.

Obama's nominee to become the next Defense Secretary, Ashton Carter, bolstered that idea during his confirmation hearing on Wednesday.

Carter said he was "very much inclined" to support sending what the administration calls "lethal" defensive arms to the Ukraine government.

"We need to support the Ukrainians in defending themselves," he said.

Such a step could take the worst East-West confrontation since the Cold War into a new and unpredictable stage.

In effect, Washington would be sending weapons to be used against Russian forces at a time when the Kremlin is increasingly hostile to the West and has shown it is ready to escalate a showdown.

There is no guarantee that funneling sophisticated U.S. armaments into the conflict could be decisive, and the gambit could antagonize Vladimir Putin , Russia's increasingly isolated and hard-to-read leader. In a sign of possible administration push back against Carter's remarks, the top NATO general Philip Breedlove told the Associated Press in an interview on Thursday that arming Ukraine could spark a "more strident" reaction from Russia.

White House spokesman Josh Earnest meanwhile offered what appeared to be a rebuke of Carter on Wednesday, saying "a decision like this will be made by the Commander-in-Chief," in a reminder of where power over national security lies in this administration.

The United States also wants to avoid a complete breakdown in ties and a return to the great power competition of the Soviet era, partly because it needs Russian help on key strategic issues, including nuclear talks with Iran, the chaos in the Middle East and in Afghanistan.

Eugene Rumer, who served as national intelligence officer for Russia and Eurasia at the US National Intelligence Council from 2010 until last year, said that arming Ukraine would prolong a conflict Kiev cannot win outright.

Photos: Crisis in Ukraine 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. Hide Caption 1 of 49 Photos: Crisis in Ukraine 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. Hide Caption 2 of 49 Photos: Crisis in Ukraine A man with a machine gun is among the Ukrainian troops standing guard in Krimskoe town of Luhansk, Ukraine, on June 25. Hide Caption 3 of 49 Photos: Crisis in Ukraine Shelling between Ukrainian troops and pro-Russian rebels leaves damage in Donetsk, Ukraine, on Monday, June 1. Hide Caption 4 of 49 Photos: Crisis in Ukraine A Ukrainian serviceman fires a grenade launcher on the front lines near Donetsk on Saturday, May 30. Hide Caption 5 of 49 Photos: Crisis in Ukraine Pro-Russian rebels carry the coffin of prominent separatist commander Alexei Mozgovoi during his funeral in Alchevsk, Ukraine, on Wednesday, May 27. Hide Caption 6 of 49 Photos: Crisis in Ukraine 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. Hide Caption 7 of 49 Photos: Crisis in Ukraine Residents flee with salvaged belongings during renewed fighting in the Jabunki neighborhood near the airport in Donetsk on Monday, April 13. Hide Caption 8 of 49 Photos: Crisis in Ukraine 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. Hide Caption 9 of 49 Photos: Crisis in Ukraine An instructor of the Ukrainian volunteer Azov Battalion conducts training exercises in Kiev, Ukraine, on Sunday, March 1. Hide Caption 10 of 49 Photos: Crisis in Ukraine A woman makes her way across a bridge destroyed in heavy fighting in Donetsk, Ukraine, on March 1. Hide Caption 11 of 49 Photos: Crisis in Ukraine A volunteer gives humanitarian aid to residents of Popasna, Ukraine, on Saturday, February 28. Hide Caption 12 of 49 Photos: Crisis in Ukraine A Ukrainian serviceman climbs out of a tank at a checkpoint near Horlivka, Ukraine, on Monday, February 23. Hide Caption 13 of 49 Photos: Crisis in 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. Hide Caption 14 of 49 Photos: Crisis in Ukraine 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. Hide Caption 15 of 49 Photos: Crisis in Ukraine Pro-Russian rebels stationed in Horlivka launch missiles on Wednesday, February 18. Hide Caption 16 of 49 Photos: Crisis in Ukraine Pro-Russian separatists take position near Uglegorsk, Ukraine, on February 18. Hide Caption 17 of 49 Photos: Crisis in Ukraine An army ambulance damaged in recent shelling lies by a road near Svitlodarsk, Ukraine, on Sunday, February 15. Hide Caption 18 of 49 Photos: Crisis in Ukraine Ukrainian servicemen play with a soccer ball on a road between Svitlodarsk and Debaltseve on February 15. Hide Caption 19 of 49 Photos: Crisis in Ukraine A woman salvages items February 15 from the rubble of a destroyed clinic where she had worked in Opytne, Ukraine. Hide Caption 20 of 49 Photos: Crisis in Ukraine People carry a refrigerator through a balcony at an apartment building that was damaged in recent shelling in Svitlodarsk on February 15. Hide Caption 21 of 49 Photos: Crisis in Ukraine 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. Hide Caption 22 of 49 Photos: Crisis in Ukraine People stand beside the body of a woman killed during shelling in Kramatorsk, Ukraine, on Tuesday, February 10. Hide Caption 23 of 49 Photos: Crisis in Ukraine A volunteer gets a medical checkup at a military base for pro-Russian rebels February 10 in Donetsk, Ukraine. Hide Caption 24 of 49 Photos: Crisis in Ukraine 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. Hide Caption 25 of 49 Photos: Crisis in Ukraine Residents unload humanitarian aid in Debaltseve on Friday, February 6. Hide Caption 26 of 49 Photos: Crisis in Ukraine A man rides a bicycle in Vuhlehirsk, Ukraine, on February 6. Hide Caption 27 of 49 Photos: Crisis in Ukraine A child waits on a bus to leave Debaltseve on Tuesday, February 3, after increased fighting in the region. Hide Caption 28 of 49 Photos: Crisis in Ukraine A man stands next to his car in Donetsk on Sunday, February 1, after it was destroyed by shelling. Hide Caption 29 of 49 Photos: Crisis in Ukraine The body of a civilian killed during shelling lies on the ground in Donetsk on Friday, January 30. Hide Caption 30 of 49 Photos: Crisis in Ukraine People in Mariupol, Ukraine, pour soil into the grave of a recent shelling victim on Monday, January 26. Hide Caption 31 of 49 Photos: Crisis in Ukraine Ukrainian servicemen prepare ammunition at a position on the front line near Mariupol on January 26. Hide Caption 32 of 49 Photos: Crisis in Ukraine A man injured during shelling in Mariupol sits in an emergency hospital on January 26. Hide Caption 33 of 49 Photos: Crisis in Ukraine 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. Hide Caption 34 of 49 Photos: Crisis in Ukraine A resident walks by a burning building in Mariupol on Saturday, January 24. Hide Caption 35 of 49 Photos: Crisis in Ukraine A pro-Russian rebel takes cover from shelling in the Kievsky district of Donetsk on Thursday, January 22. Hide Caption 36 of 49 Photos: Crisis in Ukraine People in downtown Donetsk react as Ukrainian prisoners of war are handed over by pro-Russian rebels on January 22. Hide Caption 37 of 49 Photos: Crisis in Ukraine A trolleybus is damaged in Donetsk's Lenin District after its station was hit by a shell on January 22. Hide Caption 38 of 49 Photos: Crisis in Ukraine A rebel takes aim while protecting a supply position in the Kievsky district of Donetsk on January 22. Hide Caption 39 of 49 Photos: Crisis in Ukraine Rubble and debris cover the airport in Donetsk on Wednesday, January 21. Hide Caption 40 of 49 Photos: Crisis in Ukraine Hide Caption 41 of 49 Photos: Crisis in Ukraine 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. Hide Caption 42 of 49 Photos: Crisis in Ukraine Women sit in a shelter during a battle in Donetsk on Sunday, January 18. Hide Caption 43 of 49 Photos: Crisis in Ukraine A building hit by Ukrainian artillery is seen in the Voroshilovsky area of Donetsk on January 18. Hide Caption 44 of 49 Photos: Crisis in Ukraine Men from the Azov Volunteer Battalion board a bus in Kiev to join the fight against the rebels on Saturday, January 17. Hide Caption 45 of 49 Photos: Crisis in Ukraine Rebels sit atop a tank at a checkpoint north of Luhansk, Ukraine, on Wednesday, January 14. Hide Caption 46 of 49 Photos: Crisis in 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. Hide Caption 47 of 49 Photos: Crisis in Ukraine 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. Hide Caption 48 of 49 Photos: Crisis in Ukraine A Ukrainian volunteer fighter fires a machine gun at pro-Russian rebels near the village of Pisky, Ukraine, on Saturday, January 3. Hide Caption 49 of 49

"This proposal will not deter Putin. So it is both insufficient but it also is dangerous," Rumer said. "Dangerous because it could take us in a direction that we have not thought about and those consequences are quite real."

He went on:"Putin has so far been willing and demonstrated an ability to escalate despite the breakdown in relations with Europe and the U.S., despite the economic fallout, despite the sanctions, despite the price of oil."

But the idea of arming Ukraine is nonetheless on the table because its armed forces have suffered heavy defeats in eastern regions to separatists which have been well supplied by Moscow and apparently bolstered by Russian troops.

Secretary of State John Kerry significantly hardened the U.S. tone on the conflict after arriving in Kiev on Thursday to meet President Petro Poroshenko.

"We cannot close our eyes to tanks that are crossing the border from Russia and coming into Ukraine, we cannot close our eyes to Russian fighters in unmarked uniforms crossing the borders, and leading individual companies of so-called separatists into battle," Kerry said.

The top US diplomat also said that modern weapons, including rockets were flowing across the border into eastern Ukraine in breach of a peace deal signed by Russian, the Kiev government and separatists last year. Since then Russian-backed forces have seized large swathes of territory on eastern Ukraine.

Since the start of the conflict, the U.S. has sent military aid including night vision goggles to Ukraine, but stopped short of sending lethal items. But Kerry and National Security Advisor Susan Rice are now open to considering a change of policy, officials tell CNN. The aim would be to increase the military cost for Russia's activity in Ukraine in a bid to convince the Kremlin to seek a diplomatic way out.

The push to arm Ukraine got another boost this week with a report by a group of eight influential foreign policy scholars -- including several former senior Obama appointees. The group called on Obama to spend $3 billion over three years to arm Ukraine with lethal defensive weapons.

Obama insists that the Western sanctions, which have hammered Russia's economy, are exacting a painful cost for Russia's moves in Ukraine.

But while that price is high, it has yet to change Putin's behavior in a conflict which started with the annexation of Crimea last year. That reality is fueling calls on both sides of the aisle in Congress for tougher US action.

"The only thing that deters Putin is a show of strength," Democratic Rep. Adam Schiff told CNN on Thursday.

Republican Sen. Kelly Ayotte, meanwhile, said Putin was a "schoolyard bully" picking on the weakest member of the class. "It's time to provide Ukraine the ability to defend itself," she said.

Administration officials insist that no decisions have yet been made to increase U.S. assistance to Ukraine. White House officials declined to comment on the president's deliberations for this story.

Those urging Obama to go ahead with an effort to arm Ukraine argue that he must make the point that flagrant violations of international order perpetrated by Putin in Ukraine are unacceptable.

"A person like Putin understands power," said Ivo Daalder, Obama's former ambassador to NATO, who joined the group of prominent scholars, who also included former Clinton deputy secretary of state Strobe Talbott and Michele Flournoy, who is often spoken about as a possible secretary of defense for Hillary Clinton.

"Our unwillingness even to contemplate providing defensive weapons sends a signal that we are not particularly serious about Ukraine's capacity to defend itself or in upholding these fundamental rules of international order," Daalder said.

The group called on Obama to provide equipment including drones, secure communications facilities, armored Humvees, as well as light anti-armor missiles, to target Russian armored vehicles.

"Only if the Kremlin knows that the risks and the costs of further military action are high, will it seek to find an acceptable political solution," the report warned.

The authors said that the West faced a critical moment with Russia -- saying that if Moscow succeeds in dismembering Ukraine, it could be emboldened to undermine the security order elsewhere in Europe.

"It might tempt President Putin to use his doctrine of protecting ethnic Russians and Russian speakers in seeking territorial changes elsewhere in the neighborhood, including in the Baltic states, provoking a direct challenge with NATO," the report warned.

The new moment of confrontation with the Kremlin was also reflected in Obama's new national security strategy which was released on Friday.

"We will continue to impose significant costs on Russia through sanctions and other means while countering Moscow's deceptive propaganda with the unvarnished truth. We will deter Russian aggression, remain alert to its strategic capabilities, and help our allies and partners resist Russian coercion over the long term, if necessary," the White House strategy said.

The serious deterioration of Russia-U.S. relations can be seen by comparing the document to the previous national security strategy, released in 2010, which speaks of cooperation with "allies and partners around the world, including Russia and touts the achievements of Obama's first term "reset" of relations with the Kremlin.

As he ponders whether to risk worsening the relationship with Russia still further by arming Ukraine, Obama could face resistance on the world stage.

German Chancellor Angela Merkel, the most powerful European leader, has already made clear Germany will not supply weapons to Kiev. Merkel, who will meet Obama at the White House on Monday, arrived in Moscow on Friday with French President Francois Hollande, seeking to convince Putin to sign on to a ceasefire in eastern Ukraine and a long term peace plan.

Some Russian experts believe a move to arm Ukraine could backfire, and play into Putin's conspiratorial mindset.

Sean Kay, an expert on Russia and NATO at Ohio Wesleyan University said that Putin could also swiftly increase Russian assistance to rebels in a bid to lock in quick battlefield gains. He said it would be six months to a year before a U.S. operation could have a measurable effect on the situation on the ground in Ukraine.

"The moment Putin thinks this weaponry is coming in, he could dramatically escalate things in a very dangerous way to take advantage of the current window," he said.

