Barack Obama has accused Russia of sending troops into Ukraine and being responsible for a surge in violence in the country.

Speaking at a news conference in Washington, the US president said Russia was encouraging, training, arming and funding separatists in the region and warned Moscow that it faced further isolation.

He said: "Russia has deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine and the new images of Russian forces inside Ukraine make that plain for the world to see. This comes as Ukrainian forces are making progress against the separatists."

Obama again ruled out US military action, but claimed Russia's increasing involvement in Ukraine "will only bring more costs and consequences for Russia", threatening a further tightening of sanctions.

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

Obama said: "As a result of the actions Russia has already taken, and the major sanctions we've imposed with our European and international partners, Russia is already more isolated that at any time since the end of the cold war. Capital is fleeing. Investors are increasingly staying out. Its economy is in decline."

He added that he would reaffirm the US's "unwavering commitment to … Ukraine and its people" when President Petro Poroshenko visited the White House in September.

"We are not taking military action to solve the Ukrainian problem. What we're doing is to mobilise the international community to apply pressure on Russia," he told reporters.

Obama's comments come after photographs showed Russian soldiers in uniform and Russian weapons in action in a renewed offensive against Ukrainian troops.

At an emergency meeting of the UN security council Britain's ambassador, Mark Lyall Grant, accused Russia of deploying more than 1,000 troops.

"Formed units of the armed forces of the Russian Federation are now directly engaged in fighting inside Ukraine against the armed forces of Ukraine. These units consist of well over 1,000 regular Russian troops equipped with armoured vehicles, artillery and air defence systems," he said.

A state department spokeswoman, Jen Psaki, amplified Obama's comments with details of Russia's involvement in Ukraine.

"Russia has ... stepped up its presence in eastern Ukraine and intervened directly with combat forces, armoured vehicles, artillery, and surface-to-air systems, and is actively fighting Ukrainian forces as well as playing a direct supporting role to the separatists' proxies and mercenaries," she told a media briefing.

US ambassador Samantha Power accused Russia of lying about its involvement in Ukraine. "It has manipulated. It has obfuscated. It has outright lied," she said.

"The mask is coming off. In these acts, these recent acts, we see Russia's actions for what they are: a deliberate effort to support, and now fight alongside, illegal separatists in another sovereign country."

Russia's UN ambassador, Vitaly Churkin, responded: "There are Russian volunteers in eastern parts of Ukraine. No one is hiding that."

Moscow has said some Russians have, in their own time, gone to Ukraine to support the cause of the separatists.

He urged the US to "stop interfering in the internal affairs of sovereign states."

The Russian president, Vladimir Putin, has yet to respond directly to Obama's accusations. But he called on pro-Moscow separatists in Ukraine to open a humanitarian corridor to allow besieged Ukrainian troops to retreat.

"I call on the rebel forces to open a humanitarian corridor for the Ukrainian troops who are surrounded, so as to avoid unnecessary casualties and to give them the opportunity to withdraw from the zone of operations," Putin said in a statement.

Encircled Ukrainian fighters have been engaged in a fight for survival in the town of Ilovaysk for more than a week as pro-Russian rebels, who had been on the retreat, staged a swift counter-offensive.

Putin addressed the separatists directly as defenders of "Novorossiya", or New Russia.

Putin called on Kiev to "immediately halt military action, cease fire, sit down at the negotiating table and speak with representatives of the Donbass, and solve all of the problems which have built up exclusively by peaceful means".

The Russian president called on the rebels to provide wounded Ukrainian soldiers with medical help and said Russia was "ready and will provide humanitarian aid to the people of the Donbass who are suffering from this humanitarian catastrophe".

Last week Russia sent into eastern Ukraine a convoy of more than 200 lorries, which it said was carrying more than 1,800 tonnes of aid, without the permission of Kiev and without Red Cross monitors.

The unilateral move was condemned by Kiev and the west, who were concerned an inadvertent attack on the official convoy might serve as a pretext for Russia to send in troops.