MOSCOW (REUTERS) - US Secretary of State John Kerry urged Russian Foreign Minister Sergei Lavrov on Wednesday to stop Russian and separatist attacks on Ukrainian forces in Debaltseve, State Department spokeswoman Jen Psaki told reporters.

"Secretary Kerry spoke with Foreign Minister Lavrov this morning. He pressed Foreign Minister Lavrov to stop Russian and separatist attacks on Ukrainian positions in Debaltseve and other violations of the ceasefire," Psaki said.

"The secretary urged Russia to secure access to Debaltseve for OSCE monitors, who have been blocked from performing their responsibilities."

Ukraine pulled thousands of troops out of the encircled town of Debaltseve on Wednesday after a massive assault by pro-Russian rebels, who ignored a new ceasefire to seize the strategic railway junction.

The fall of Debaltseve in eastern Ukraine was one of the worst defeats of the war for Ukraine's troops, hindering their ability to stop an advance by Moscow-backed rebels fighting for territory the Kremlin calls "New Russia".

Despite the fighting, the US State Department maintained that the ceasefire negotiated last week in the Belarussian capital of Minsk was not "dead."

"We don't consider it dead," Psaki said.

"We still need to give time for the agreement to work through."