U.S. President Barack Obama stepped up pressure on Russia over Syria on Monday, telling his Russian counterpart Vladimir Putin of his concern about the reported use of chemical weapons by the Damascus regime.

Obama also thanked Putin in the telephone call for his help after the Boston marathon bombings, and expressed condolences over a fire that killed 36 patients in a Russian psychiatric facility on Friday, the White House said.

Russian President Vladimir Putin will host his U.S. counterpart Barack Obama for a bilateral visit later this year as the two leaders pledged to intensify cooperation on Syria and counter- terrorism, a Kremlin aide said on Monday.

Putin's foreign policy aide Yury Ushakov said Obama had called Putin to discuss future contacts with the Russian leader including a bilateral visit just before the G20 summit that Russia hosts in Saint Petersburg in early September.

Japanese Prime Minister Shinzo Abe and Russian President Vladimir Putin on Monday pledged to renew efforts to find a solution to a decades-long territorial row that has prevented the two sides from signing a World War II peace treaty.

After several hours of talks in the Kremlin, Abe and Putin agreed to order their foreign ministers to reopen talks on finding options for a solution that could be presented to the leaders.

Russian President Vladimir Putin expressed hope that the Boston bombing tragedy would result in closer cooperation between Moscow and Washington in the fight against terror.

"I hope this tragedy pushes us closer to one another in stopping shared threats," Putin said during his live televised call-in session in Moscow, saying Russia was also a victim of "international terrorism".

A Moscow court on Thursday jailed a prominent Russian activist for two-and-a-half years over his involvement in an opposition protest last year on the eve of Vladimir Putin's inauguration for a third Kremlin term.

Konstantin Lebedev, an aide to the leader of the radical Left Front Sergei Udaltsov, was convicted of organizing a mass riot and jailed by the Moscow city court, Russian news agencies reported.

Russia and the United States agreed Saturday to step up cooperation in their fight against terror in the wake of news that two ethnic Chechens were suspected of organizing the deadly Boston Marathon bombings.

The Kremlin said Russian leader Vladimir Putin called U.S. President Barack Obama to once again express his condolences and discuss ways the two sides can work more closely on security in the runup to the 2014 Winter Olympic Games in Sochi.

Russia and Egypt called for a ceasefire in Syria "as quickly as possible", President Vladimir Putin said Friday, after a meeting with his Egyptian counterpart Mohamed Morsi.

"Russia and Egypt support a ceasefire in Syria as soon as possible and the launch of negotiations," between the regime and rebels, the Russian president said following the meeting in the black city resort of Sochi, where the 2014 Winter Olympic Games will be held.

The Kremlin warned Washington on Friday against publishing a list of Russian officials barred from entering the United States because of their alleged abuse of human rights.

Russian media reports said the dozens of names included on the so-called "Magnitsky Act" -- named after Russian lawyer Sergei Magnitsky who died in prison after revealing a suspected state embezzlement scheme -- would be published in the United States later on Friday.

Russia on Wednesday vowed to extend economically struggling Serbia a $500-million loan while reasserting support for its ally's continuing refusal to recognize the independence of Kosovo.

Serbian Prime Minister Ivica Dacic won a promise of the quick disbursement of a $300-million (230-million-euro) loan that would help cover Belgrade's budget shortfall.

Russian President Vladimir Putin on Monday welcomed a US decision to delay the test of an international ballistic missile to avoid stoking tensions with North Korea.

"I think the United States took a very important step in delaying the test of a ballistic missile," he told reporters at an industrial fair in Germany, speaking through an interpreter.