ANKARA (Reuters) - Seventy-one officers, including six generals, have defected from the Syrian army to Turkey, a Turkish official said on Saturday, the biggest single mass desertion of senior soldiers from President Bashar al-Assad's security forces in months.

It was not immediately clear why the group had deserted.

Just hours ago, the United States said it would arm Syrian rebels, having obtained proof that Assad's forces used chemical weapons against fighters trying to end the president's rule.

(Reporting by Jonathon Burch; Writing by Seda Sezer; Editing by Nick Tattersall and Louise Ireland)