Russia said on Tuesday that a U.S. decision to ease some 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 government forces in a conflict with rebels, some of whom are supported by the United States.
Foreign Ministry spokeswoman Maria Zakharova said the policy change easing some restrictions on weapons supplies to rebels was set out in a new U.S. defence spending bill and that Moscow regarded the step as a hostile act.
U.S. President Barack Obama signed the annual defence policy bill into law last week.
"In the administration of B. Obama they must understand that any weapons handed over will quickly end up in the hands of jihadists with whom the sham 'moderate' opposition have long acted jointly," Zakharova said in a statement.
"Such a decision is a direct threat to the Russian air force, to other Russian military personnel, and to our embassy in Syria, which has come under fire more than once. We therefore view the step as a hostile one."
Zakharova accused the Obama administration of trying to "put a mine" under the incoming administration of President-elect Donald Trump by attempting to get it to continue what she called Washington's "anti-Russian line."
During his election campaign, Trump said he was keen to try to improve relations with Moscow and spoke positively about President Vladimir Putin's leadership skills.
A back-and-forth exchange between Trump and Putin over nuclear weapons last week tested the Republican's promises to improve relations with Russia.
The Obama administration and U.S. intelligence officials have accused Russia of trying to interfere with the U.S. election by hacking Democratic Party accounts.
(Reporting by Andrew Osborn/Peter Hobson; Editing by Raissa Kasolowsky)