WASHINGTON (BLOOMBERG) - Separatist leaders from two self-declared republics in eastern Ukraine have appealed to Russian President Vladimir Putin for assistance repelling Ukrainian forces, the Kremlin said, a potential step toward Moscow sending forces there.

Kremlin spokesman Dmitry Peskov said the appeal, which cited allegedly increasing "aggression" by Ukrainian forces against the breakaway areas, came in a letter on Wednesday (Feb 23), Tass and Interfax reported.

Ukraine denies it plans to retake the Donbass territories by force, but ceasefire violations on both sides have picked up in recent days along the line of contact dividing their forces.

The separatist leaders' appeal to Russia for military assistance is a further escalation of the security situation, Ukraine's foreign minister Dmytro Kuleba said.

He said he had demanded an urgent meeting of the UN Security Council regarding the appeal.

“Ukraine has requested an urgent meeting of the UN Security Council due to the appeal by Russian occupation administrations in Donetsk and Luhansk to Russia with a request to provide them with military assistance, which is a further escalation of the security situation,” Kuleba said on Twitter.

A United Nations Security Council meeting has been scheduled for 9.30pm (10.30am Thursday Singapore time), diplomatic sources said.

Russia's parliament has formally given Putin the authority to deploy troops to the separatist regions after he officially recognised them this week. The Russian president has said he did not yet intend to send what he called "peacekeepers" to eastern Ukraine, but would do so "as necessary".

Ukraine and its Western allies charge that Russian forces have already been in the territories since their formation in 2014. Moscow denies that.