The U.S. aircraft carrier Carl Vinson on Monday made a port call at Vietnam's central city of Danang, facing the contentious South China Sea, marking the first visit of its kind since the Vietnam War ended in 1975.

The four-day visit by the nuclear-powered carrier reflects growing strategic ties between the former adversaries and is seen as a veiled message to China, which has disputes with Vietnam and other Southeast Asian countries over islands and reefs in the South China Sea.

It underscores the dramatic improvement of overall relations between the United States and Vietnam since communist forces defeated the U.S.-backed South Vietnamese government more than four decades ago and reunified the country.

"The visit marks an enormously significant milestone in our bilateral relations and demonstrates U.S. support for a strong, prosperous and independent Vietnam," U.S. Ambassador Daniel Kritenbrink said in a statement, adding that the two countries have "gone from former enemies to close partners."

Prior to its port call in Danang, the U.S. Navy's third Nimitz-class super-carrier was in the Philippines, which has also rows with China over disputed features in the South China Sea in recent years.

When U.S. President Donald Trump visited Vietnam last December and met with his Vietnamese counterpart Tran Dai Quang, they welcomed the planned carrier visit and reaffirmed the importance of freedom of navigation and unfettered commerce in the South China Sea, as well as a rules-based approach to resolving maritime disputes.

That was followed up in January when U.S. Defense Secretary Jim Mattis met in Hanoi with Vietnamese Defense Minister Ngo Xuan Lich, at which time he said the two countries are "like-minded partners" with common interests including respect for international law.

The two sides committed to enhance defense cooperation based on a three-year plan of action agreed upon last October, with a focus on maritime security, humanitarian assistance and disaster relief, as well as peacekeeping operations.

At the time of Mattis' visit, Joint Staff Director Lt. Gen. Kenneth McKenzie said that carrier visits "send a reassuring signal to allies and regional partners...and they send a signal also to those who don't wish us well."

The United States has taken issue with China's assertive moves to press its claims in the South China Sea, including the creation and expansion of military installations, partly through land reclamation.

In the sea, Vietnam and China are contesting the territorial rights over the Spratly and Paracel islands.

The United States and Vietnam normalized relations in 1995.