President Barack Obama underscored U.S. concerns about Syrian chemical weapons in a phone conversation with Russian President Vladimir Putin on Monday, the White House said.

The United States is trying to determine the facts around alleged Syrian use of chemical weapons. Last week U.S. officials said they had “varying degrees of confidence” that such weapons were used in Syria, which if proven with certainty could triggered unspecified actions against the Syrian government.

“President Obama and President Putin reviewed the situation in Syria, with President Obama underscoring concern over Syrian chemical weapons,” a White House statement said.

The two leaders agreed to stay in touch on the issue and that Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov would continue discussions on Syria.

Obama and Putin are to meet face to face twice in coming months – at a G8 summit in Northern Ireland in June and again in September at a U.S.-Russia summit in Russia.

(Reuters)