At least 10 civilians were killed in renewed fighting that broke out in Donetsk, according to authorities. The military says a Ukraine warplane was shot down over the Luhansk region. (Reuters)

Ukrainian rebels are receiving new armored vehicles and fighters trained in Russia, help that they plan to use to launch a major counter-offensive against government forces, a separatist leader said in a video released Saturday.

The four-month conflict in eastern Ukraine has reached a critical phase, with Kiev and Western governments watching nervously to see whether Russia will intervene in support of the increasingly besieged rebels — an intention Moscow denies.

Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People’s Republic, said the rebels were in the process of receiving about 150 armored vehicles, including 30 tanks, and 1,200 fighters who he said spent four months training in Russia.

“They are joining at the most crucial moment,” he said in a video recorded Friday. He did not specify where the vehicles would come from.

Moscow has been the target of heavy Western sanctions over its annexation of Ukraine’s Crimea and accusations it is supporting separatists in eastern Ukraine with fighters, arms and funds. Russia denies those charges.

In a sign of concern at the latest rebel comments, German Chancellor Angela Merkel and Ukrainian President Petro Poroshenko agreed in a phone call Saturday that deliveries of weapons to separatists in Ukraine must stop and a cease-fire must be achieved, a German government spokesman said.

The risk of outright war between the two most powerful former Soviet states was highlighted Friday when Ukraine said it had 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, instead saying it was a “fantasy” that its armored vehicles had entered its neighbor’s territory.

Vice President Biden also spoke to Poroshenko on Saturday, and the White House said: “The two leaders agreed that Russia’s sending military columns across the border into Ukraine and its continued provision of advanced weapons to the separatists was inconsistent with any desire to improve the humanitarian situation in eastern Ukraine.”

Ukrainian Foreign Minister Pavlo Klimkin called on NATO to provide military support for Ukrainian troops.

The rebels, who have ceded ground to government forces in recent weeks, have been promising a counter-offensive for several days but have yet to launch one.

Zakharchenko, a Ukrainian native, took over from Russian citizen Alexander Borodai on Aug. 7, and his combative comments will probably dash hopes that changes at the top of the rebel leadership might signal willingness to end hostilities.

Pro-Russian rebels park a tank on their base near the town of Krasnodon, eastern Ukraine, Saturday, Aug. 16, 2014. (Sergei Grits/AP)

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 12 miles from the Ukrainian border, unmoved since Friday.

Officials from the International Committee of the Red Cross said most procedures had been agreed to 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 reached.

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 that an estimated 2,086 people have been killed, with nearly 5,000 wounded.

Finnish President Sauli Niinisto held talks in Kiev with Poroshenko, a day after discussing how to settle the crisis with Russian President Vladimir Putin.

“I do not see a great risk of an outright war,” Niinisto said. “My hopefulness is based on the fact that communication is open, at least by a crack.”

France said a meeting of Ukrainian, Russian, German and French foreign ministers scheduled in Berlin on Sunday could be a first step toward a peace summit.

— Reuters