US President Barack Obama accused Russia of training and arming Ukraine's separatists. He said images proved that the country had stationed forces inside Ukraine. He warned Russia not to risk further isolation.

"They're doing this to themselves," Obama told reporters at the White House on Thursday.

Obama said Petro Poroshenko would visit the US next month, his first trip to Washington as Ukraine's president. Obama also said he had spoken by phone with Angela Merkel about the issue on Thursday.

The US envoy to the UN, Samantha Power, told the Security Council that Russia had sent soldiers, tanks and weaponry to support separatists. The accusation came during an emergency session called after NATO reported that hundreds of Russian troops had crossed into eastern Ukraine to shore up separatists.

'An increased presence'

On Thursday afternoon, Chancellor Merkel said that EU ministers were scheduled to meet Saturday to discuss the issue. Ukrainian officials say Russian military columns rolled into the country after firing missiles at a border post.

"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," Merkel told a news conference on Thursday after talks with Balkan leaders in Berlin.

UN Secretary General Ban Ki-Moon has called reports of firing in Ukraine a "dangerous escalation," according to his spokesman.

"The international community cannot allow the situation to escalate further nor can a continuation be allowed of the violence and destruction that the conflict has wrought in eastern Ukraine," UN spokesman Stephane Dujarric told reporters on Thursday.

Russian officials have rejected claims that the military is stationed in Ukraine. A Defense Ministry spokesman quoted by the Interfax news agency said that the allegations bore "no relation to reality."

The North Atlantic Treaty Organization has announced that the ambassadors of its 28 members will hold an emergency meeting on Friday to discuss the situation with officials from Ukraine, which is not a member but meets regularly with the organization in a body called the NATO-Ukraine Commission.

mkg/crh (Reuters, AFP, dpa, AP)