Russian Foreign Minister Sergei Lavrov has told U.S. Secretary of State John Kerry that the fight against extremist groups in Syria must be stepped up because some militants are using poison gas to attack civilians.

Russia's Foreign Ministry said Lavrov made the remarks to Kerry on August 5 during a phone call about the Syrian war.

The two men spoke a day after Moscow sharply criticized U.S. actions in Syria, accusing Washington of backing rebels who use poison gas against civilians.

U.S. President Barack Obama said on August 4 that he was not sure he can trust Russia or Russian President Vladimir Putin to cooperate with the United States in the fight against extremists in Syria, but was exploring the possibilities.

He said Washington must "test whether or not we can get an actual cessation of hostilities that includes an end to the kinds of aerial bombing and civilian death and destruction that we have seen carried out by the [Syrian] regime."

Based on reporting by Reuters, AP, AFP, TASS, and Interfax