In a three-way telephone conversation on Thursday, German Chancellor Angela Merkel and French President Francois Hollande called on Russian President Vladimir Putin to convince pro-Russian separatists in eastern Ukraine to lay down their arms.

Merkel and Hollande warned Putin that if the Kremlin failed to play a constructive role in ending the violence in Ukraine, the international community would impose additional sanctions against Russia.

The warning came after NATO Secretary-General Anders Fogh Rasmussen said on Thursday that Russia had deployed several thousand additional troops to the Ukrainian border, calling it "a very regrettable step backward."

Merkel and Hollande also called on Putin to resume negotiations with Ukraine over Kyiv's outstanding natural gas debt and the price of future deliveries. Talks broke down last weekend, with Moscow demanding payment from Ukraine in advance.

Putin calls for Kyiv to end violence

During the conversation, "Mr. Putin expressed grave concern about Kyiv's continuing military operation in southeast Ukraine," according to the Kremlin.

"Hope was expressed that the Kyiv leadership's assurances of intention to end the violence and launch a national dialogue will be implemented immediately," the Kremlin said in a news release.

President Putin also spoke with his Ukrainian counterpart Petro Poroshenko by telephone on Thursday, the two leaders' second conversation in less than a week. They reportedly discussed the details of Poroshenko's peace plan, which has not yet been made public.

The Ukrainian president promised on Wednesday to declare a unilateral cease-fire, giving pro-Russian separatists in eastern Ukraine the chance to lay down their weapons.

Reports of heavy casualties

Heavy fighting was reported in eastern Ukraine on Thursday near the city of Krasnyi Liman in the Donetsk region. A spokesman for the government forces, Vladislav Seleznev, described the area as an important supply route for the pro-Russian rebels.

Four Ukrainian soldiers were killed and another 20 were wounded in the fighting, according to Seleznev. He said that 200 rebels died in the battle, a claim that has not been independently verified.

Rebel sources in Donetsk said that the Ukrainian infantry, backed by 20 tanks and other armored vehicles, had stormed the village of Yampil some 12 kilometers (seven miles) east of Krasnyi Liman. A top rebel commander in the region, Igor Strelkov, reported "heavy losses"

"We beat off the first attack and destroyed one tank," Strelkov told Reuters news agency. "Our people are holding but we can't rule out that they [government forces] will break through."

Strelkov rebuked Moscow for not coming to the rebels' aid, saying he hoped "they have enough conscience left in Moscow to take some measures."

slk/jm (AP, AFP, Reuters)