Baghdad ? Four U.S. helicopters flew into Syrian airspace Sunday afternoon and opened fire, killing eight people near the Iraqi border, the Syrian government said.

The reported operation in al-Boukamal, roughly six miles from the Iraqi border, occurred at approximately 4:45 p.m., the Syrian Arab News Agency reported, citing an unnamed government source.

Reported U.S. attacks inside Syria are extremely rare, though the U.S. military has stepped up security along Iraq’s border with Syria in recent months to stem the traffic of fighters and weapons into Iraq. U.S. officials say many insurgents, particularly suicide bombers, arrive in Iraq via the Syrian border.

The U.S. military in Baghdad did not immediately respond to an inquiry about the reported operation. A spokeswoman for the State Department, Joanne Moore, declined to comment.