Protesters try to stop municipal workers and volunteers from clearing away their tents Saturday on Kiev's Independence Square, the scene of street protests that toppled a Moscow-backed president in February. (Konstantin Chernichkin/Reuters)

In a move that may open the door to a de-escalation of the grinding conflict in Ukraine, Ukrainian President Petro Poroshenko on Saturday said he was ready to accept an international humanitarian mission into the easternmost part of his nation.

The announcement came as a top separatist leader said that the key rebel-held city of Donetsk had been surrounded by the Ukrain­ian military and that he was seeking a cease-fire. There was no immediate Ukrainian response.

On a day of conversations between top Ukrainian, U.S., Russian and German officials, the developments may point the way toward a negotiated easing of the battle over rebel-held portions of eastern Ukraine. Ukrainian government forces have made rapid strides in recent weeks to retake rebel-held territory, but insurgents still control key cities, and urban combat has threatened to be bloody and difficult for both sides.

As the rebels have come under pressure, U.S., NATO and Ukrain­ian officials have been warning with increasing urgency that Russian forces may be poised to come into Ukrainian territory under the pretext of a peacekeeping deployment. Russian diplomats have been seeking United Nations backing for a humanitarian mission in Ukraine, but Western officials have been cautious about interfering as Kiev forces turn back the rebels. About 20,000 Russian troops are on the border, NATO said this week.

In a conversation with German Chancellor Angela Merkel, Poroshenko said that “we are ready to receive humanitarian assistance” in the easternmost region of Luhansk, according to a readout of the conversation. But he said that it would have to be international, that it would need to pass through Ukrainian-
controlled border checkpoints, and that it would go without military escort apart from Ukrainian-military-provided security.

Volunteers in Kiev on Saturday remove the last of the barricades and tent camps that had blocked the main street of Ukraine's capital since protests arose late last year, leading to a change of government. (Efrem Lukatsky/AP)

Merkel has been the primary negotiator between the West and Russian President Vladimir Putin. The two last spoke Wednesday, according to the Kremlin.

The Ukrainian government accused Russia of trying to send a column of vehicles into Ukraine early Saturday morning under the guise of a humanitarian mission. Kremlin spokesman Dmitri Peskov denied the Ukrainian account, but said that the situation in Ukraine was the “number-one issue for discussion,” the Interfax news service reported.

Secretary of State John F. Kerry on Saturday told Russian Foreign Minister Sergey Lavrov that “Russia should not intervene in Ukraine under the guise of humanitarian convoys or any other pretext of ‘peacekeeping,’ ” the State Department said in a statement.

President Obama also spoke Saturday to Merkel.

On the ground in eastern Ukraine, fighting continued Saturday, and separatist leaders said the rebel stronghold of Donetsk had been surrounded. The Ukrainian military did not immediatelyconfirm whether it had fullyencircled the city.

“We are ready for a cease-fire to prevent the spreading of a humanitarian disaster,” a top rebel leader, Alexander Zakharchenko, said in a statement.

Also Saturday, Kiev continued to clear the encampment of protesters who have occupied central Independence Square since November. The protests led to the February ouster of pro-Russian President Viktor Yanukovych. Russia annexed the Crimean Peninsula in March, and pro-Russian rebels began seizing territory in eastern Ukraine in April.