World powers have called a succession of emergency meetings to step up the international response to Russia after Kiev accused Moscow of a de facto invasion and of opening up a second front in the conflict in eastern Europe.

The UN security council was meeting in emergency session, the US national security council convened at the White House, and Nato and EU leaders will consider their response on Friday, amid signs that hundreds of Russian soldiers are actively involved in the insurrection against Kiev's rule in Ukraine.

Russia denies that any of its troops are in eastern Ukraine. But on Thursday Nato said it estimated there were now more than 1,000 Russian soldiers fighting there. The organisation released satellite images that it said showed Russian armoured vehicles and artillery had been crossing into Ukraine for at least a week.

A satellite image showing what Nato claims are self-propelled Russian artillery units inside Ukraine. Photograph: Nato/DigitalGlobe/EPA

Ukraine's president, Petro Poroshenko, said: "Russian forces have actually entered Ukraine," while Ukrainian fighters in the south-east said Russians had helped separatists take over Novoazovsk, a border town.

Western leaders swiftly seized on the latest escalation by warning the Russian president, Vladimir Putin, of imminent consequences.

At a hastily convened press briefing, Barack Obama said the US expects to agree a further round of economic sanctions against Russia during meetings with European allies next week, but has ruled out any military intervention to deter further attacks on Ukraine.

Angela Merkel, the German chancellor, said the EU would discuss further sanctions at a summit this weekend, particularly in the light of the fresh incursion in which Russia stands accused of funnelling troops and hardware into the south-east of Ukraine. "We are getting reports of an increased presence of Russian soldiers and of new unrest and fresh advances of the separatists in areas that until now were very quiet," she said. "We made it clear in March this year that if there were a further escalation, more sanctions would have to be discussed."

Obama played down the extent to which reports of direct Russian attacks inside Ukraine marked a distinct escalation. Instead, he insisted that Ukrainian successes against Russian-backed rebels in eastern Ukraine had merely forced the Kremlin to be more "overt" in its support.

And despite growing international concern that Russian forces have effectively invaded and annexed eastern Ukraine much as they did in Crimea, Obama insisted his policy of responding with economic sanctions was working and isolating Russia more than any time since the end of the cold war.

"I think the sanctions that we have already applied have been effective, our intelligence shows the Russians know this … and there are ways for us to deepen and expand the scope of some of that work," he said. "We are not taking military action to solve the Ukrainian problem. What we are doing is to mobilise the international community to apply pressure on Russia."

A spokesman for Ban Ki-moon, the UN secretary general, said the latest allegations pointed to a "dangerous escalation" and added: "The international community cannot allow it to escalate further."

David Cameron said that if Russia did not desist "then she should be in no doubt that there will be further consequences".

He added: "I'm extremely concerned by mounting evidence that Russian troops have made large-scale incursions into southeastern Ukraine, completely disregarding the sovereignty of a neighbour.

"The international community has already warned Russia that such provocative actions would be completely unacceptable and illegal. We urge Russia to pursue a different path and to find a political solution to this crisis. If Russia does not, then she should be in no doubt that there will be further consequences."

Western powers have already imposed a slew of sanctions on Moscow that have started to show signs of hurting Russia's economy. But Putin has responded in kind and, despite meeting Poroshenko for talks on the crisis in Minsk this week, has shown no signs of changing tack.

"Recent Russian actions clearly demonstrate that Moscow is bluntly drawing Ukraine and the entire world into a full-scale war," Ukraine's foreign ministry said after Poroshenko effectively accused Russia of mounting an invasion.

"Russian forces have actually entered Ukraine," he said, ordering an urgent meeting of Ukraine's national security council. "I have made a decision to cancel my working visit to the republic of Turkey due to sharp aggravation of the situation in Donetsk region, particularly in Amvrosiivka and Starobeshevo, as Russian troops were actually brought into Ukraine," he said. "The president must stay in Kiev today."

Addressing the council, he called on Ukrainians not to panic, and said the situation in the country's east was "difficult but controllable". He also said consultations between the Russian and Ukrainian army HQs, as agreed in Minsk, had begun, mainly to talk about prisoner exchanges.

A separatist leader admitted on Thursday that there were serving Russian soldiers among his fighters. A Russian rights group said about 100 Russian soldiers had been killed in Ukraine. Even members of Putin's human rights advisory council conceded evidence pointed to a Russian invasion.

"When masses of people, under commanders' orders, on tanks, APCs and with the use of heavy weapons, [are] on the territory of another country, cross the border, I consider this an invasion," Ella Polyakova told Reuters. She and another rights council member also said they believed that about 100 Russian soldiers had died earlier in the month when their convoy was hit by Grad missiles near the town of Snizhne in eastern Ukraine. There was no immediate way to confirm the figure.

However, Russian officials continued to deny there was any kind of invasion. The defence ministry told Russian agencies that reports of Russian military units acting in Ukraine were "fake".

"It feels like Kiev really needs a 'Russian armed invasion'," wrote Maria Zakharova, a spokeswoman for the Russian foreign ministry, on Facebook. "It seems this is part of its big plan for scaring its own population, and the main answer to the question people there have is: 'What on Earth has happened to us?' It's scary to think what statements the Ukrainian officials will come up with on Halloween."

Russia has repeatedly denied it is fighting in Ukraine, and speaking after the Minsk meeting, Putin said a solution to the crisis is "not our business; it is a domestic matter for Ukraine itself". He said all Russia could do was "support the creation of an environment of trust".

Putin's words, however, jar sharply with events on the ground. Fighters and weapons have long been able to move freely along the unguarded sections of Russia's border with Ukraine, and reporters in Novoazovsk say that what appears to be hastily repainted Russian military hardware has appeared in the town in recent days.

Russia's denials appear increasingly flimsy. When the Guardian saw a Russian armoured column cross the border two weeks ago, the foreign ministry and local security services denied an incursion had taken place, saying it was a border patrol that had strayed into Ukrainian territory. This week, when Russian paratroopers were captured well inside Ukraine, defence ministry sources said they had been part of a border patrol that had got lost and entered Ukraine "by accident".

The head of the self-declared Donetsk People's Republic, Alexander Zakharchenko, said on Thursday that there were serving Russian soldiers with his fighters, but claimed they were volunteers who were taking a holiday in the region. "Among the Russian volunteers there are many former soldiers, who are fighting alongside us and understand that it's their duty," said Zakharchenko on Russian TV. "And moreover, I'll say it openly, we also have current soldiers, who decided to take their holidays not on the beach, but among us."

On Friday night Putin called on pro-Russia rebels to provide a humanitarian corridor in east Ukraine for encircled Ukrainian government troops to leave the battlefield. "I call on the militia forces to open a humanitarian corridor for encircled Ukraine servicemen in order to avoid pointless victims, to allow them leave the fighting area without impediment, join their families … to provide urgent medical aid to those wounded as a result of the military operation," he said in a statement.

Although Putin's actions in Ukraine have been supported by most Russians, there is growing dissent in the country over mounting evidence of a stealth war. Mikhail Khodorkovsky, formerly Russia's richest man – released from jail late last year – said Russians should not be silent about their soldiers being killed in Ukraine and secretly buried. "We are fighting in Ukraine, fighting for real," he wrote. "Our authorities are always lying about this, in the 80s about Afghanistan, in the 90s about Chechnya and now about Ukraine. Why are we remaining silent? Have we become cowards? Are we scared of even thinking now?"

There have also been appeals from relatives of the soldiers captured in Ukraine, calling on Putin and the defence ministry to help bring the men home alive.

Kiev on Thursday called on its western allies for more support. The foreign ministry said: "Under the current circumstances, Ukraine counts on serious support from its international partners and believes that strengthening EU sanctions against Russia as well as providing Ukraine with military and technical support will help deter Russian aggression."