HANOI (Reuters) - Two U.S. warships made port calls at Vietnam's strategic naval base at Cam Ranh Bay, the U.S. navy said on Tuesday, in a brief but symbolic return for U.S. combat vessels to what was a crucial logistics complex during the Vietnam War.

Submarine tender USS Frank Cable and guided-missile destroyer USS John S. McCain made the visit on Sunday to the deep-water naval base, marking an important step in fast-growing defense ties between the two former enemies.

The visit came after the full lifting of a U.S. embargo on trade in lethal arms with Vietnam in May as part of President Barack Obama's strategic "rebalance" toward Asia.

At the same time, tension has been rising over rival territorial claims in the South China Sea, straining ties between Vietnam and its giant neighbor China.

Vietnam has been intensifying efforts to diversify relations in Europe and Asia and engage more with the United States.

Cam Ranh is the jewel in the crown of Vietnam's military, with an air base once used by the U.S. and Soviet forces and a deep water bay home to its modern, Russian-built submarines.

Visits by foreign ships are rare and usually reserved for maintenance. Japanese and French warships have recently made port calls at Cam Ranh.

Established as a base by the United States during the Vietnam War, Cam Ranh Bay had been used largely by Russian forces since then.

The John S. McCain visited nearby Danang city before sailing to Cam Ranh Bay, the U.S. Navy said.

(Corrects first two paragraphs to remove erroneous references to 21-year milestone.)

(Reporting by Mai Nguyen; Editing by Alison Williams)