WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis said on Tuesday that recent strikes against pro-Syrian government forces in the past few weeks had been in self-defense and the United States would take all measures to protect its forces in Syria. Last week the United States shot down a pro-Syrian government drone that fired toward U.S.-led coalition forces in Syria, but “hit dirt” and caused no injuries. On the same day, the U.S. hit two pro-Syrian government pick up trucks near the southern town of At Tanf. Russia said on Saturday it had told the United States it was unacceptable for Washington to strike pro-government forces in Syria. 