Russian forces in Syria on Wednesday accused U.S. pilots of committing "gross violations" of protocols intended to avoid a clash between the two superpowers, Russia's state news agency Sputnik said.

The agency quoted the head of the Russian reconciliation center in Syria, Admiral Oleg Gorynov, as saying that the American pilots activated their weapons systems as they approached Russian planes over the east of the country.

Russia intervened in the Syrian war in 2015, turning the tide in favor of President Bashar al-Assad, and Moscow has since expanded its military facilities in the country with a permanent air base, which also has a naval base.

The United States leads a coalition against Islamic State and has carried out air strikes in Syria this year.

Gorynov said Russia and the United States had agreed on special zones where the coalition could operate, but U.S. troops had been spotted outside those areas.

In March, Gorynov protested "provocative actions" by US forces in Syria.