WASHINGTON - US President Barack Obama and his Russian counterpart Vladimir Putin on Monday discussed the protracted conflict in Syria and agreed to have a summit in Russia in September.

In a phone conversation, the two leaders discussed the situation in Syria, where the conflict has entered its third year and killed more than 70,000 people, the White House said in a statement.

Obama underscored his concern over Syria's chemical weapons, as the United States joined Britain, France and Israel last week to allege that such weapons have been used in internal fightings in the Middle East country.

Washington is seeking more clear facts before taking further action, though Obama has called the use of or failure to secure such weapons by the Syrian government a "game changer" in his judgment about the Syrian conflict.

The two presidents agreed to "stay in close consultation" and instructed their foreign ministers to continue discussions on Syria, according to the White House.

They also said that they looked forward to meeting in person in June when leaders of the Group of Eight industrialized nations meet in Northern Ireland and again in September for a bilateral summit in Russia.

Obama reiterated his appreciation for the close cooperation the United States has received from Russia following the Boston Marathon bombing on April 15, when two suspects of Chechen origin detonated two home-made bombs near the finish line of the race, killing three people and injuring about 200 others.

"The two leaders discussed cooperation on counterterrorism and security issues going forward, including with respect to the 2014 Olympics in Sochi," a Black Sea resort in Russia, the White House noted.

It added that Obama also conveyed his condolences over the tragic mental hospital fire outside of Moscow that killed dozens last week.