Russian officials are condemning a move by the United States to speed the delivery of weapons to Syrian fighters, calling it a “hostile act” that threatens Russian forces and could lead to terrorist attacks on airplanes. The criticism follows President Obama’s signature of a massive Pentagon spending bill, known as the NDAA, which eases restrictions on weapons transfers to Syrian opposition groups. Russia says that will allow for the U.S. to arm rebels with shoulder-mounted anti-aircraft missiles known as MANPADS. At the State Department, spokesperson Mark Toner said the Obama administration is opposed to such a move.

Mark Toner: “We’ve seen this before, this language before, this kind of rhetoric. I mean, our position on MANPADS has not changed. We’re—we would have a very deep concern about that type of weaponry getting into Syria.”

Despite those comments, the incoming Trump administration could choose to arm Syrian rebels with MANPADS. Critics say the weapons could be used to down commercial jetliners. Last year, Amnesty International reported that much of the arsenal held by ISIS comes from U.S.-made weapons acquired from the Iraqi Army and Syrian opposition groups.