The U.S. Embassy in Hanoi said it is unaware of any response from the Vietnamese government to its request that it stop allowing Russia to use a base to refuel bombers that have conducted provocative flights around U.S. territory in the Pacific.
The State Department late last week communicated its concerns privately to the Vietnamese government, embassy press officer Lisa Wishman told VOA on Thursday.
The embassy official said that although the U.S. government respects Vietnam's right to enter agreements with other countries, it wants Vietnam to ensure Cam Ranh Bay is not utilized by the Russian military "to conduct activities that could raise tensions in the region."
Both Russia and Vietnam, defense partners for decades, have previously acknowledged Il-78 tanker aircraft were using Cam Ranh to refuel Tupolev Tu-95 "Bear" strategic bombers, which have an un-refueled range of 15,000 kilometers.
The former U.S. base, which also in the past was controlled by the French and Japanese navies, is a natural deep-water harbor located 290 kilometers northeast of Ho Chi Minh City (formerly Saigon).
Vincent Brooks, commanding general of the U.S. Army Pacific, told the Reuters news agency on Wednesday that Russian planes had conducted "provocative" flights, including around the U.S. Pacific Ocean territory of Guam, home to Andersen Air Force Base, which is 4,000 kilometers east of Vietnam.
NATO officials say the Bear bombers have also been spotted recently over the English Channel as Russia's military conducts more frequent and aggressive air and maritime patrols close to borders of NATO states.
Russia announced last November it planned to send long-range bombers on patrols over North American waters, an echo of the Cold War era.
The RIA news agency on Thursday, citing the Defense Ministry in Moscow, said the Russian military had begun military drills in southern Russia, the Georgian breakaway regions of South Ossetia and Abkhazia, and in Crimea, which was annexed from Ukraine last year.
The United States fought the North Vietnamese from the early 1960s until the American withdrawal in 1973, which led to the fall of the South Vietnamese government in Saigon two years later.
Diplomatic ties between Hanoi and Washington were established in 1995.
Despite strong ties between Hanoi and Moscow dating back to the Soviet era, the Vietnamese and Americans have grown closer in recent years as Hanoi grows increasingly worried about the threat posed by its huge neighbor to the north, China.
For centuries, parts of Vietnam chafed under Chinese influence. The two countries fought their most recent border war in 1979, although it was a brief conflict an estimated 30,000 soldiers on both sides died. The two countries have overlapping territorial claims in the South China Sea.
U.S.-Vietnamese joint humanitarian exercises were held in late 2014 and more are occurring this month.
The United States has also agreed to provide at least five modern patrol ships to Vietnam with delivery expected next year, part of an $18 million package U.S. Secretary of State John Kerry announced in 2013 to help Vietnam strengthen in maritime security capabilities.
In a speech at Hanoi University last Friday, U.S. Ambassador to Vietnam Ted Osius expressed understanding that Hanoi would look to "historic partners" for security. But he said the United States also had "much to offer... to enhance Vietnam's security in the short, medium and long term."