Russian troops have been “brought into Ukraine”.

So says the top man in Kyiv, Petro Poroshenko, canceling a trip to Turkey for the inauguration of its new president and calling an urgent meeting of Ukraine’s national security and defence council.

“The place of the Ukrainian president is in Kyiv,” he said.

Poroshenko is accusing Russian troops of deployment in the southeast of Ukraine in support of pro-Moscow separatist rebels.

Kyiv, whose forces had been making gains, says the town of Novoazovsk has fallen under the control of Russian forces who, together with the rebels, are staging a counter-offensive.

This is a new front, away from the rebel-held stronghold of Donestk and close to Ukraine’s strategic port city of Mariupol. Some reports suggest rebels are already advancing there.

Russia denies sending soldiers into Ukraine but a rebel leader says some had volunteered.

“We never hid the fact that there are many Russian citizens among us, without whose help it would be very difficult to fight now,” said separatist leader Alexander Zakharchenko .

“There were about 3,000- 4,000 of them in our ranks. Many have left but more are still here. There are also many serving soldiers who preferred to spend their vacation with us, brothers who are fighting for their freedom, rather than on a beach.”

The latest escalation in the crisis comes just days after the presidents of both countries held their first talks in more than two months and agreed to work towards launching a peace process.