NOVOAZOVSK, Ukraine — Ukraine accused Russia on Thursday of entering its territory with tanks, artillery and troops, and Western powers said Moscow had "outright lied" about its role and dangerously escalated the conflict.
Russia dismissed the allegations, describing the fighters there as "Russian volunteers." The Kremlin has repeatedly denied arming and supporting the separatists who have been fighting Ukrainian troops for four months in the gravest crisis between Russia and the West since the end of the Cold War.
NATO said at least 1,000 Russian troops are in Ukraine and later released what it said were satellite photos of Russian self-propelled artillery units moving last week.
Two columns of tanks and other equipment entered southeastern Ukraine at midday, following heavy shelling of the area from Russia that forced overmatched Ukrainian border guards to flee, said Col. Andriy Lysenko, a spokesman for Ukraine's national security council.
"Russian forces have entered Ukraine," President Petro Poroshenko said in Kiev, canceling a foreign trip and calling an emergency meeting of his security council.
He urged Ukrainians to remain calm.
"Destabilization of the situation and panic, this is as much of a weapon of the enemy as tanks," Poroshenko told the council.
U.S. President Barack Obama spoke with German Chancellor Angela Merkel, and both leaders agreed Russia must face consequences for its actions.
"We agree — if there was ever any doubt — that Russia is responsible for the violence in eastern Ukraine," Obama said. "The violence is encouraged by Russia. The separatists are trained by Russia, they are armed by Russia, they are funded by Russia."
He added that 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."
But Obama ruled out a military confrontation between the U.S. and Russia. He said Russia's activity in Ukraine would incur "more costs and consequences," though these seemed to be limited to economic pressure that will be discussed when Obama meets with European leaders at a NATO summit in Wales next week.
At an emergency session of the U.N. Security Council, Western representatives expressed outrage.
"Now we see irrefutable evidence of regular Russian forces operating inside Ukraine," said British Ambassador Mark Lyall Grant.
U.S. Ambassador Samantha Power said Russia "has manipulated. It has obfuscated. It has outright lied."
Russian Ambassador Vitaly Churkin offered a spirited defense, saying Kiev "is waging war against its own people."
He did not deny the Russian presence, saying "there are Russian volunteers in eastern parts of Ukraine. No one is hiding that."
But Power countered: "A Russian soldier who chooses to fight in Ukraine on summer break is still a Russian soldier."
Churkin questioned the presence of Western advisers and asked where Ukrainian troops were getting weapons. He said he wanted to "send a message to Washington: Stop interfering in the internal affairs of sovereign states."