Russia said on Tuesday that a U.S. decision to ease restrictions on arming Syrian rebels had opened the way for deliveries of shoulder-fired anti-aircraft missiles, a move it said would directly threaten Russian forces in Syria.

Moscow last year launched a campaign of air strikes in Syria to help President Bashar al-Assad and his forces retake territory lost to rebels, some of whom are supported by the United States.

Foreign Ministry spokeswoman Maria Zakharova said the policy change easing restrictions on weapons supplies had been set out in a new U.S. defense spending bill and that Moscow regarded the step as a hostile act.

U.S. President Barack Obama, who has been sharply critical of Russia’s intervention in Syria, signed the annual defense policy bill into law last week.