Pro-Russian separatists shot down a Ukrainian warplane in eastern Ukraine early yesterday while at least 10 civilians were killed in fighting involving artillery and mortars in the rebel-held city of Donetsk, local authorities said.
The renewed fighting, which followed a claim by rebels that they were receiving fresh stocks of heavy military equipment from Russia and 1,200 fighters, cast a shadow over a high-level international meeting in Berlin involving Russia and Ukraine.
The four-month conflict in Ukraine's Russian-speaking east has reached a critical phase, with Kiev and Western governments watching nervously to see if Russia will intervene in support of the increasingly besieged rebels - something denied by Moscow.
A Ukrainian military spokesman said the rebels had hit a MiG-29 fighter jet over the Luhansk region, one of two regions where entrenched separatists were fighting a rearguard action to hold off government forces advancing on their positions.
The pilot ejected and was located and recovered after a search, spokesman Oleksiy Dmytrashkivsky said.
Donetsk, the region's biggest city where rebels are also still dug in despite an encroaching government offensive, was rocked by heavy weapons throughout the night. At least 10 civilians had been killed, local authorities said.
The Ukrainian National Guard said its forces had seized a rebel field commander from Luhansk region as well as 13 others suspected of "terrorist activity".
"The terrorists are putting on ordinary clothes, taking only their passport with them and are trying to pass themselves off as ordinary peaceful citizens on public transport to try to get through the Ukrainian checkpoints," it said. "Among those seized is a field commander of the Luhansk terrorist group."
On Saturday, Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People's Republic, said rebels were in the process of receiving some 150 armoured vehicles, including 30 tanks, and 1,200 trained fighters with which they planned to launch a major counter-offensive against government forces.
"They are joining at the most crucial moment," he said in a video recorded on Friday. He did not specify where the vehicles would come from.
The renewed fighting and rebel claims of reinforcements from Russia were certain to be broached at a Berlin meeting of Ukrainian, Russian, German and French foreign ministers scheduled for late yesterday. France said the meeting could be a first step towards a peace summit.
In a sign of concern at the latest rebel comments, German Chancellor Angela Merkel and Ukrainian President Petro Poroshenko agreed in a phone call that deliveries of weapons to separatists in Ukraine must stop and a ceasefire must be achieved, a German government spokesman said.
The risk of outright war between the two most powerful former Soviet states was highlighted on Friday when Ukraine said it had partially destroyed an armoured column that had crossed the border from Russia.
But Moscow made no threat of retaliation and dismissed as a "fantasy" the assertion that its armoured vehicles had entered its neighbour's territory.
The United Nations said this month that an estimated 2,086 people, both civilians and combatants, had been killed in the four-month conflict. That figure has nearly doubled since the end of July, when Ukrainian forces stepped up their offensive and fighting started in urban areas.