Ukrainian rebels say they are ready for a ceasefire.

KIEV (Reuters) Pro-Russian separatists declared on Saturday that they were ready for a ceasefire with the Kiev government in the wake of increasing gains by Ukrainian forces over rebel forces.

"We are ready for a ceasefire to prevent the spread of a humanitarian disaster in Donbass," said Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People's Republic, in a statement, referring to the area in eastern Ukraine where fighting is taking place.

He warned that Donetsk, the country's industrial hub and center of the rebel resistance, is facing shortages of food, water, and electricity, but said the rebels are ready to defend the city of about one million people.

"In the event of an offensive in the city, the number of victims will increase significantly. We have no humanitarian corridors. There is no supply of medicine... food stocks are almost depleted," he said.

Ukrainian authorities have stated they are ready to agree to a ceasefire, but on the condition that the rebels surrender their weapons.

The office of Ukrainian President Petro Poroshenko was not immediately available for comment on Zakharchenko's statement.

Earlier, Kyiv declared it had thwarted a Russian attempt to send troops to Ukraine disguised as peacekeeping forces with the aim of provoking a large-scale military conflict, a claim Moscow dismissed as a "fairy tale."

Ukraine has made several similar statements about Russian aggression during months of conflict with separatists, which it says are supported by Moscow, on its eastern border with Russia, none of which can be independently verified.

Ukrainians claim to be gradually closing the net around the rebels, who have been forced to return to their strongholds in Donetsk and Luhansk on the border.

(By Pavel Polityuk in Kyiv)