A Russian intelligence warship docked in Havana on Tuesday, a day before the start of historic U.S.-Cuba talks aimed at normalizing diplomatic relations.

There was nothing stealthy about the arrival of the Viktor Leonov CCB-175, which was moored to a pier in Old Havana where cruise ships often dock. But the visit was not officially announced by Cuban authorities.

The foreign ministers of Ukraine, Russia, Germany and France are set to meet Wednesday in Berlin in a bid to de-escalate the Ukraine conflict, Germany said.

"The chief aim now is to prevent a further deterioration of the military conflict and a renewed political escalation between Kiev and Moscow. This is worth every effort," German Foreign Minister Frank-Walter Steinmeier said in a statement.

Russian-backed insurgents on Tuesday assaulted the remnants of a Ukrainian force hanging onto a ruined airport near the rebels' main stronghold as Moscow poured cold water on the prospect of peace talks.

Blasts of incoming and outgoing artillery echoed all night across Donetsk -- a once bustling industrial city but now the crucible of one of Europe's worst humanitarian and diplomatic crises since the Cold War.

France arrested five Russians Tuesday accused of plotting a new attack as four men suspected of helping the gunmen behind the Paris shootings were brought before anti-terrorist judge.

European nations, on high alert after the attacks that shook France to its core, have launched a wave of raids targeting suspected jihadist cells.

Russia on Tuesday denied as "absolute nonsense" the latest accusations by Kiev that Moscow had sent hundreds of troops over the border into Ukraine.

"Yesterday's accusations from Kiev of the alleged deployment of Russian army contingents across the Russian-Ukrainian border do not stand up to any criticism," defense ministry spokesman Igor Konashenkov said, Russian news agencies reported.

Russian Defense Minister Sergei Shoigu signed a military cooperation deal with Iran on Tuesday that his Iranian counterpart touted as a joint response to U.S. "interference".

Shoigu is the most senior Russian military official to visit Tehran since 2002, according to Iranian media, and the agreement comes with both countries facing Western sanctions.

EU foreign ministers agreed Monday there would be no change in the bloc's Russia policy, including sanctions, as the union's diplomatic chief warned the Ukraine situation had got "much worse" in recent weeks.

They said they would stick to their course until Russia fully implemented the Ukraine peace accords it had backed in September, despite EU foreign policy chief Federica Mogherini's suggestion of a review in the way the European Union deals with Russia.

Hundreds of Russians marched in Moscow and Saint Petersburg on Monday, six years after a lawyer and reporter were murdered by ultra-nationalists.

About 500 people walked through central Moscow mourning slain lawyer Stanislav Markelov and Anastasia Baburova, a journalist with an opposition paper, who were gunned down on a central Moscow street in broad daylight on January 19, 2009.

Sporadic shelling hit a hospital in Donetsk and shook other parts of the region on Monday while Ukraine alleged that hundreds of Russian troops crossed the border to help rebels fighting for control of the country's east.

Ukraine's fresh charges of Russia backing the rebels comes after each side blamed the other for a complete breakdown of a September truce in recent days, with heavy combat having rocked the area around Donetsk airport.

Fighting eased in the Donetsk region on Monday after Ukraine launched a major offensive and claimed to clear pro-Russian rebels from most of the airport outside the key eastern city.

There were a number of sporadic explosions overnight, but the violence seemed to be far less than in previous days, when heavy clashes shook the area around the airport and shelling moved closer toward the city center, killing at least 10 civilians.