By Natalia Zinets and Thomas Grove
KIEV/DONETSK (Reuters) - Pro-Russian separatists shot down a Ukrainian warplane in eastern Ukraine overnight into Sunday while at least 10 civilians were killed in fighting involving artillery and mortar 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 are fighting a rear-guard action to hold off government forces advancing on their positions.
The pilot ejected and was located and recovered after a search, the spokesman, Oleksiy Dmytrashkivsky, told Reuters.
Donetsk, the region's biggest city where rebels are also still dug in despite an encroaching government offensive, rocked to the crash and boom of heavy weapons in the south throughout the night.
At least 10 civilians have 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 armored 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 was certain to be broached at a meeting scheduled in Berlin later on Sunday of Ukrainian, Russian, German and French foreign ministers.
France said the meeting could be a first step towards a peace summit.
Moscow has come under heavy Western sanctions over its annexation of Ukraine's Crimea and accusations it is supporting the separatists with fighters, arms and funds. Russia denies those charges.
SIGN OF CONCERN
In a sign of concern at the latest rebel comments, German Chancellor Angela Merkel and Ukrainian President Petro Poroshenko agreed in a phone call on Saturday 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 partially destroyed an armored column that had crossed the border from Russia. The report triggered a sell-off in global shares.
But Moscow made no threat of retaliation and dismissed as a "fantasy" the assertion that its armored vehicles had entered its neighbor's territory.
The United Nations said this month that an estimated 2,086 people, including civilians and combatants, had been killed in the four-month conflict. That figure nearly doubled since the end of July, when Ukrainian forces stepped up their offensive and fighting started in urban areas.
In Donetsk which is now ringed by Kiev's forces, artillery fire has struck apartment buildings, killing and wounding residents. Officials in Kiev deny they are firing heavy weapons at residential areas.
Adding to the tensions, Russia and Ukraine have been at loggerheads for days over a convoy of 280 Russian trucks carrying water, food and medicine, which remained about 20 km (12 miles) from the Ukrainian border, unmoved since Friday.
Officials from the International Committee of the Red Cross said most procedures had been agreed by Russia and Ukraine but the two sides still needed to figure out how to provide security before the convoy moves ahead under the ICRC's aegis. It was not clear when a deal on security could be agreed.
A Reuters journalist reported from the Russian settlement of Kamensk-Shakhtinsky, where the convoy has been waiting, that 16 trucks from the convoy had left the marshalling yard and headed in the direction of the Ukrainian border.
The Ukrainian government said late on Saturday it had designated the Russian convoy as humanitarian aid, in principle clearing it to cross the border under the auspices of the Red Cross. The Russia column comprises about 280 trucks in total.
Russia says it is a purely humanitarian mission in support of civilians in areas hit by the conflict, but Ukraine is concerned it could serve as a 'Trojan horse' to infiltrate military supplies or create a pretext for armed intervention.
The crisis has dragged relations between Russia and the West to their lowest point since the Cold War and set off a round of trade restrictions that are hurting struggling economies in both Russia and Europe. The United Nations said this week an estimated 2,086 people had been killed and nearly 5,000 wounded.
The momentum on the ground is with the Ukrainian forces, who have pushed the separatists out of large swathes of territory and nearly encircled them in Donetsk and Luhansk. Kiev says it now controls the road linking the two cities, though heavy fighting on that road raged overnight into Sunday.
Russia says the Ukrainian offensive is causing a humanitarian catastrophe for the civilian population in the two cities. It accuses Kiev's forces of indiscriminately using heavy weapons in residential areas, an allegation Ukraine denies.
In the past week, three senior rebel leaders have been removed from their posts, pointing to mounting disagreement over how to turn the tide of the fighting back in their favor.
(Writing By Richard Balmforth; Editing by Christian Lowe and Ralph Boulton)