Russia has provisionally agreed to reopen a major Cold War listening post on Cuba that was used to spy on the United States, a Russian daily reported Wednesday after President Vladimir Putin visited the island last week.

Kommersant reported that Russia and Cuba had agreed "in principle" to reopen the Lourdes base, mothballed since 2001, citing several sources within Russian authorities.

Mediators on the Ukraine crisis have accused pro-Russian rebels of not being committed to truce negotiations after a planned video-conference to discuss talks fell through.

A discussion meant to be held Tuesday between the trilateral Contact Group -- which includes representatives of Ukraine, Russia and the OSCE -- and separatist leaders "did not materialize" meaning that no talks have happened since late June, the group said in a statement.

Russian investigators said Wednesday they had detained two metro workers over a devastating crash in the Moscow subway that killed at least 21 people, as anger mounted over the tragedy.

City authorities declared Wednesday a day of mourning after the worst accident in the metro's eight-decade history, while Russians swapped stories of how the authorities had ignored their complaints about the famed transport system.

A new round of EU sanctions against Russia and pro-Moscow separatists in Ukraine was "looking very possible", a diplomatic source said Tuesday on the eve of an EU summit where leaders would make the final decision.

According to the source, member states were preparing a variety of new measures, including freezing programmes in Russia run by the EU's European Investment Bank and the London-based European Bank of Reconstruction and Development.

Ukraine's Western-backed leaders on Tuesday invited pro-Kremlin insurgents to a video-conference aimed at halting spiraling violence and what Kiev has warned was an imminent invasion by thousands of Russian troops.

Kiev sharply raised the stakes in Europe's most explosive crisis in decades by declaring on Monday that a Ukrainian transport plane downed in the eastern conflict zone had been hit by a rocket fired from the Russian side of the frontier between the two ex-Soviet states.

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.

Ukraine, Russia, Germany and France called for fresh ceasefire talks on east Ukraine by Tuesday at the latest, Berlin's chief diplomat said, as nerves started to fray in the volatile region.

Germany's Foreign Minister Frank-Walter Steinmeier held phone talks with his French counterpart Laurent Fabius, Ukrainian Foreign Minister Pavlo Klimkin and Kremlin chief of staff Sergei Ivanov, according to a Berlin statement on Monday.

German and Russian support for U.S. President Barack Obama has nosedived in the past year over revelations of spying and tensions in Ukraine, an influential American research group said Monday.

And while the United States remained relatively popular, the Pew Research Center said it found widespread opposition to U.S. eavesdropping as well as to the Pentagon's use of drones overseas.

Russia's Foreign Minister Sergei Lavrov has invited the OSCE to send observers to monitor the Russian-Ukrainian border, the ministry said Monday, in a bid to defuse escalating tensions in east Ukraine.

"As a gesture of goodwill, and without waiting for the enforcement of a ceasefire regime, the Russian side is inviting OSCE observers to the Gukovo and Donetsk border posts on the Russian-Ukrainian border," a ministry statement said.

The Kremlin denied on Monday a report that Moscow was considering targeted strikes against Ukraine after shells landed across the border and killed a Russian civilian.

Respected daily Kommersant had quoted a source close to the Kremlin as saying Moscow was considering "targeted retaliatory strikes" against Ukraine, where clashes between pro-Moscow rebels and government troops escalated over the weekend.