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.

Three Ukrainian soldiers have been injured in a massive all-night attack carried out by hundreds of pro-Russian insurgents in the nation's restive east, authorities in Kiev said Wednesday.

The assault on a position held by the Ukrainian National Guard in the Lugansk region began Tuesday evening and lasted 10 hours, the interior ministry said, adding that six rebels were also killed in the fighting.

Russia said Monday it was ready to talk to Ukraine's new president Petro Poroshenko, raising hopes that after months of tensions Moscow sees in the billionaire a leader it can work with.

"We are ready for dialogue with Kiev's representatives, with Petro Poroshenko," Foreign Minister Sergei Lavrov said in Russia's first reaction to his victory in Sunday's vote.

With a bloody insurgency raging in the east, the ominous presence of Russian troops across the border and an economy in deep recession, next Sunday's election will determine the very survival of Ukraine.

But it is unclear whether a large chunk of the population will want to -- or be able to -- turn out to choose a new president for a country many fear is on the brink of civil war.

The United States and European partners are ready to impose tougher sanctions against Russia if it seeks to disrupt Ukraine's upcoming presidential elections, top diplomats warned Thursday.

Ukrainian officials are also working to hold another round of unity talks in eastern Ukraine in the coming days, U.S. Secretary of State John Kerry said after talks in London.

Several explosions and gunfire could be heard near the flashpoint eastern Ukrainian town of Slavyansk Monday, according to an Agence France Presse reporter there, as Kiev seeks to flush out pro-Moscow rebels holed up there.

Fighting broke out in a nearby village on the "front line" of the siege of the town of at least 110,000 people amid what Kiev calls an "anti-terrorist" operation, a rebel spokeswoman said.

Several Red Cross members whom rebels said they had detained in the eastern Ukrainian city of Donetsk have been released, an official from the organization in Kiev told Agence France Presse Saturday.

"An emergency response team from the local Red Cross and one member of the international Red Cross were detained. They are now all free," said the official, who did not wish to be named.

Ukraine's military launched assaults to retake rebel-held eastern towns on Thursday in which up to five people were reported killed, a move Russian President Vladimir Putin warned would have "consequences."

The offensive sent international tensions soaring and oil prices up on the prospect of Russia making good on its threat of a massive response in the ex-Soviet republic.

Ukraine will demand that Russia cease its support for "terrorist activities" on its territory, Kiev's foreign minister said Wednesday on the eve of crunch talks with Moscow, the U.S. and EU.

"Our main demand is to de-escalate the situation in eastern Ukraine. We want Russia to withdraw the troops from the eastern borders of Ukraine. We want Russia not to support terrorist activities in eastern Ukraine," Foreign Minister Andriy Deshchytsya said after he landed in Geneva for Thursday's meeting.

Russian Foreign Minister Sergei Lavrov is to meet Chinese President Xi Jinping in Beijing next week and will discuss with him the situation in Ukraine, the Russian foreign ministry said Sunday.

Lavrov will visit China on Tuesday, ahead of four-way talks between diplomats from the European Union, United States, Ukraine and Russia planned in Geneva on Thursday. Those talks are now in doubt amid violent clashes between supporters of Kiev and pro-Russian protesters in eastern Ukraine.