Twenty people died and scores more were wounded after a train derailed in Moscow's packed metro during rush hour on Tuesday in the worst accident ever to hit one of the world's busiest subways.

Russian television described scenes of chaos and panic on the capital city's famed system, saying passengers fell like dominoes when the train traveling at 70 kilometers (over 40 miles) an hour braked abruptly and three carriages derailed and crumpled.

Urgent EU-brokered gas talks between Russia and Ukraine resumed in Kiev on Sunday ahead of a looming threat by Moscow to cut off its neighbor's supplies in a move that could impact Europe, an EU source told Agence France Presse.

Russia's RIA Novosti news agency cited its own source as saying that the first meeting involved EU Energy Commissioner Guenther Oettinger, Ukrainian Prime Minister Arseniy Yatsenyuk and Alexei Miller, the head of Russia's state gas firm Gazprom.

World leaders urged Vladimir Putin on Wednesday to stop destabilizing Ukraine or face further sanctions as they met without a Russian president for the first time since the 1990s.

Putin reached out a hand despite being banned from the Group of Seven summit following Russia's annexation of Crimea in March, saying that he was ready to meet Ukraine's president-elect.

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.

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.

Pro-Russia protesters disrupted a public meeting with German Chancellor Angela Merkel in Berlin Wednesday, calling for the arrest of "Nazis" in Ukraine.

Around a hundred protesters, wearing caps emblazoned with the Russian two-headed eagle emblem, booed the chancellor and brandished placards bearing slogans such as "Arrest the Nazis in Ukraine" and "No war against Russia".

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.

Some 100,000 workers joined a May Day parade on Moscow's Red Square Thursday for the first time since the 1991 Soviet breakup, as the takeover of Crimea boosts President Vladimir Putin's standing amid a surge of patriotism.

A huge column of demonstrators waving Russian flags and balloons marched through the iconic square near the Kremlin walls as trade union leaders addressed them from the podium on International Labor Day.

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.

The United States is coordinating with its European allies on "additional steps" to slap more sanctions on Russia over the crisis in Ukraine, a U.S. official said Tuesday.

Secretary of State John Kerry spoke by phone with his French, German and British counterparts Tuesday, but new sanctions are unlikely to be unveiled before crunch talks slated for Thursday in Geneva, State Department spokeswoman Jen Psaki told reporters.