US President Barack Obama told Russian President Vladimir Putin he was concerned about what he claimed was the use of chemical weapons by the Syrian regime.

US President Barack Obama told Russian President Vladimir Putin he was concerned about what he claimed was the use of chemical weapons by the Syrian regime.

According to a White House statement, Obama also thanked Putin in a 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.

"President Obama and President Putin reviewed the situation in Syria, with President Obama underscoring concern over Syrian chemical weapons," the statement said, considering this would cross a US "red line."

Obama and Putin agreed to stay in touch on Syria and asked Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov with working together on the issue.

The US president also used the call with Putin to thank Russia for its assistance in the probe into the Boston bombings two weeks ago.

"The two leaders discussed cooperation on counterterrorism and security issues going forward, including with respect to the 2014 Olympics in Sochi,"
the White House statement added.

In parallel, 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 two counterparts 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.