Washington - President Barack Obama stepped up pressure on Russia over Syria on Monday, telling President 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.
The call came with Obama facing increasing political heat himself, after the White House said last week that it believed there was growing evidence that Syrian forces had used chemical weapons in the civil war.
"President Obama and President Putin reviewed the situation in Syria, with President Obama underscoring concern over Syrian chemical weapons," a White House statement said.
Obama is under pressure because last year he said that the use or movement of chemical weapons by President Bashar Assad's embattled forces would cross a US "red line".
Key political players in Washington are now warning that his credibility is on the line, though the White House is seeking more detailed intelligence into exactly how and when such weapons may have been used.
Obama and Putin agreed to stay in touch on Syria and tasked Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov with working together on the issue.
Washington has been deeply frustrated that Russia has blocked tougher action in the UN Security Council, including new sanctions, against its long-time ally Syria.
The United States is also now calling on Assad to allow a United Nations team into the country to assess reports that chemical weapons have been used.
The UN-Arab League mediator Lakhdar Brahimi was in Washington on Monday and met both Kerry and Defence Secretary Chuck Hagel, officials said.
The talks focused on his efforts to broker an elusive political solution to the civil war in Syria, humanitarian aid to Syrian refugees and the military capabilities of the Syrian opposition.
Obama also used the call with Putin to thank Russia for its assistance in the probe into the Boston bombings two weeks ago, blamed on two brothers of Chechen origin, one of whom had raised flags with Moscow's intelligence services.
"The two leaders discussed co-operation on counterterrorism and security issues going forward, including with respect to the 2014 Olympics in Sochi," the White House statement said.
One of the brothers suspected of carrying out the Boston bombings had links with two figures in the Islamist anti-Kremlin insurgency in the Northern Caucasus, a Russian security source told AFP on Monday.
Tamerlan Tsarnaev, aged 26, who was killed in a police shootout, had been in touch with a Dagestan militant named Makhmud Nidal and a militant of Canadian origin named William Plotnikov, the source in the Northern Caucasus said.
Putin's foreign policy aide Yury Ushakov said the leaders agreed to ramp up counter-terrorism efforts in the wake of the Boston bombings.
The leaders are due to meet on the sidelines of the G8 summit in Northern Ireland in June and before the G20 summit in Saint Petersburg in September.