A ragtag army of opponents to Col. Moammar Gadhafi began moving west toward Tripoli from the east and the U.S. ordered two warships to the Mediterranean Sea, as the prospect of an extended war loomed over Libya.

A convoy of armed youth, including what appeared to be rebel military forces, was seen heading Tuesday night toward the pro-Gadhafi stronghold city of Sirte, witnesses said. The forces were viewed passing westward through Ajdabiya, a city about 75 miles from the opposition stronghold of Benghazi, said four residents,...