Moscow on Tuesday blasted the United States over the arrest of an alleged Russian spy, condemning the move as a "provocation" that would further damage already tattered ties.

"The U.S. has decided to launch the latest stage of its anti-Russian campaign," the foreign ministry said in a statement, accusing Washington of "stoking spy mania". 

Syrian opposition figures on Monday began four days of talks on how to end the near four-year conflict, with members of President Bashar Assad's regime set to join them but the main exiled opposition alliance boycotting the event.

The closed-door talks opened as planned at 11 am Moscow time (0800 GMT), a Russian diplomatic source told RIA Novosti. The talks are set to run until Thursday.

U.N. Secretary-General Ban Ki-moon condemned a deadly rocket attack in Ukraine's government-held port of Mariupol on Saturday and denounced "provocative statements" made by pro-Russian separatist fighters.

Ban's remarks came after pro-Kremlin rebels announced a major new offensive and heavy rocket fire killed at least 30 people and wounded around 100 in Mariupol, a strategic port linking separatist territory with Russian-occupied Crimea.

Ukraine on Tuesday accused Russian forces of attacking its soldiers after crossing over into the ex-Soviet state's war-wrecked separatist east in violation of a September truce deal.

The charges -- met with initial silence by Moscow -- should add further tensions to difficult talks in Berlin on Wednesday aimed at stemming a spike in fighting that has already killed 4,800 people and driven a million from their homes.

A key opposition group tolerated by Syria's regime is still studying whether to accept a Russian invitation to talks this month with government officials, dissident figure Haytham Mannaa said Wednesday.

The exiled National Coalition and leading opposition figure, Moaz al-Khatib, have already announced they will skip the talks that are aimed at finding a political solution to end Syria's nearly four-year war.

Russian authorities blocked access Tuesday to the website of leading Kremlin critic Alexei Navalny, the opposition activist announced on social media.

A spokesman for media regulator Roskomnadzor confirmed the move, telling TASS news agency that Russian prosecutors had ordered the restriction.

Moscow may ask for early payment on its three billion dollar loan to Ukraine, Russian Finance Minister Anton Siluanov said Saturday, accusing Kiev of violating loan conditions.

"Ukraine has violated the conditions of this loan, specifically that Ukraine's public debt does not surpass 60 percent of its GDP," the minister told Russian news agencies.

A key Syrian opposition official said in a statement on Friday that he has declined an invitation to take part in a Russian-led bid for new talks to end the conflict.

Moaz al-Khatib, ex-leader of the main opposition Syrian National Coalition, who recently established his own dissident movement, had visited Russia last year for discussions.

Hundreds in Washington, New York and Canada rallied Wednesday night in the frigid cold proclaiming "I am Charlie" in solidarity with those killed in a deadly attack on French magazine Charlie Hebdo.

Some 200 to 300 French and American protesters gathered in the nation's capital in front of the Newseum, a museum dedicated to news, waving signs and French flags emblazoned with the now ubiquitous rallying cry "Je suis Charlie."

Ukrainian police on Monday launched a terror probe into a blast in the government-controlled port of Odessa after an explosion outside a building housing a soldiers' support center.

Local authorities said the late Sunday explosion in the city of one million caused no injuries but shattered windows in the office of volunteers helping soldiers fighting pro-Russian insurgents in eastern Ukraine.