Russian officials lashed out at Ukraine's foreign minister Sunday after footage showed him using an expletive to describe Russian President Vladimir Putin as he sought to calm angry demonstrators attacking the Russian embassy in Kiev.

Andriy Deshchytsya was filmed by local television being cheered by protesters after telling them "Putin is a prick" in a bid to stop the crowd storming Russia's diplomatic mission Saturday.

Ukrainian security services are holding two Russian television journalists working for a defense ministry outlet after they were detained at a government checkpoint leaving the conflict-hit east of the country, the channel said Sunday. 

The journalists from Russia's Zvezda, or Star channel -- Nikita Konashenkov and Yevgeny Davydov -- were detained Saturday by Ukraine's National Guard as they headed to the airport after covering the fighting in eastern Ukraine between government forces and pro-Russian rebels. 

Russia's foreign minister urged U.S. Secretary of State John Kerry on Saturday to use his influence over Ukraine's leaders to convince them to suspend a military offensive in the separatist east.

Russian Foreign Minister Sergei Lavrov "urged the United States to use its influence with the Kiev authorities to nudge them toward a more realistic assessment of the situation, making them realize the hopelessness of using force to resolve a domestic conflict," the Russian foreign ministry said after the two diplomats spoke by telephone.

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.

Pro-Russian rebels shot down a Ukrainian military transport plane Saturday in the separatist east, killing all 49 troops on board in the biggest single loss of life in the two-month insurgency.

The attack came hours before top Moscow and Kiev officials were expected to meet in the Ukrainian capital for 11th-hour gas negotiations aimed at averting an imminent cut in Russian supplies that would also impact large swathes of Europe.

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.