About 300 protesters tore down the Russian embassy flag in Kiev on Saturday in the wake of the downing by pro-Kremlin rebels of a military transport plane that killed 49 troops.

The angry crowd -- some of them chanting "Fascists!" -- overturned several cars and pelted the embassy with eggs while a dozen city police officers looked on without interfering, an Agence France Presse correspondent said.

The leaders of France and Germany Saturday voiced "extreme concern" over the ongoing violence in Ukraine and called for a ceasefire after 49 died when pro-Russian separatists shot down an army plane.

During a telephone call with Russian President Vladimir Putin, French President Francois Hollande and German Chancellor Angela Merkel "stressed the importance of rapidly reaching a ceasefire in Ukraine," according to a statement issued by Hollande's office.

Russia has given its preliminary agreement to attend EU-brokered gas negotiations in Kiev on Saturday aimed at averting an imminent halt in deliveries, Ukraine's energy minister told AFP.

"We are hoping that these negotiations take place today in Kiev. The Russian side has given its preliminary agreement to attend," Energy Minister Yuriy Prodan said by telephone.

The United States accused Russia of sending tanks and rocket launchers to pro-Moscow rebels in Ukraine as Kiev's resurgent forces hoisted the national flag over a strategic rebel-held port amid fears of possible Russian gas cuts.

Moscow said Ukrainian tanks had crossed the border into its territory before being intercepted, a day after Ukraine alleged that three tanks had crossed from Russia into its territory, underscoring the growing tensions between Kiev and Moscow.

Interior Minister Nouhad al-Mashnouq is scheduled to travel to the Russian capital Moscow later this month for talks with various officials, reported al-Liwaa newspaper on Saturday.

It said that he is set to discuss diplomatic and security issues with his Russian counterpart Vladimir Kolokoltsev, deputy Foreign Minister Sergei Ryabkov, and the head of intelligence.

Two Ukrainian tanks crossed the border into Russia on Friday before being intercepted by the border patrol, the Kremlin said.

Kremlin spokesman Dmitry Peskov told the Interfax news agency that the incursion took place on Friday, a day after Ukraine hit out over an alleged border violation by Russian tanks.

Ukrainian forces said they had hoisted the national flag over the strategic rebel-held port of Mariupol on Friday in their biggest advance since Petro Poroshenko's election as the insurgency-wrecked country's pro-Western president.

Poroshenko celebrated the soldiers' "heroism" by proclaiming the industrial Sea of Azov city of half-a-million people the new temporary capital of Donetsk -- an eastern rustbelt region overrun by pro-Russian gunmen for the past two months.

Her eyes red, Valentina Chebanova sat opposite her seven-year-old granddaughter as a group of women and children waited to be bussed to Russia by rebels in Ukraine's southeastern city of Donetsk.

"I lived all my life here... I don't want to go to Russia forever," she said as she began to weep.

A loud explosion rang out in central Donetsk on Thursday evening followed by a series of smaller explosions that sounded like artillery, Agence France Presse journalists said.

The explosions were heard shortly after 10:00 pm local time (1900 GMT). A spokeswoman for the separatist Donetsk people's republic could not be immediately reached for comment.

Russian President Vladimir Putin held phone talks Thursday with his Ukrainian counterpart Petro Poroshenko on Kiev's plans for defusing the crisis in the separatist east.

"Ukraine's president notified Vladimir Putin of his plan to settle the situation in Ukraine's southeast," Putin's spokesman Dmitry Peskov told Russian news agencies.