WASHINGTON (Reuters) - The U.S. Navy dismissed two senior officers on Monday after a series of collisions involving Seventh Fleet warships in Asia, citing a loss of confidence in their ability to command.   Rear Admiral Charles Williams, commander of Task Force 70, and Captain Jeffrey Bennett, commander of Destroyer Squadron 15, were fired by Seventh Fleet commander Vice Admiral Phil Sawyer, the Navy said. In August, Sawyer replaced fleet commander Vice Admiral Joseph Aucoin, who was fired after the accidents.  Both reliefs were due to a loss of confidence in their ability to command,  the Navy statement said. The shakeup in the Seventh Fleet command followed a pre-dawn collision between guided-missile destroyer USS McCain and a merchant vessel east of Singapore and Malaysia on Aug. 21, which killed 10 sailors and was the fourth major incident in the U.S. Pacific Fleet this year. In June, another destroyer, the USS Fitzgerald, collided with a Philippine cargo ship, killing seven U.S. sailors. Several other officers have also been relieved, with administrative action taken against other members of the ship s watch teams. 