The United States and Laos, they said, plan to exchange military attaches by the end of the year as part of the strategy aimed at beefing up defense links with the trio in the heart of a once central Cold War battleground.
Three years after resuming full military ties with Indonesia soured by human rights concerns, "we are beginning to develop those same kind of ties with Vietnam, Laos and Cambodia," deputy US assistant secretary of state Scot Marciel said.
"We are starting off small -- doing some training, some exchanges which we think are very useful," he said.
"And by the end of this year, we and the Lao government will open defense attache offices in each other's capitals, which is a big step, an important step," Marciel, the US ambassador to the Association of Southeast Asian Nations (ASEAN), said at a Washington forum last week.
Vietnam, Laos and Cambodia were latecomers to ASEAN together with Myanmar.
The other ASEAN states are Brunei, Indonesia, Malaysia, the Philippines, Singapore and Thailand.
US officials rejected any notion that the move to build military ties with Vietnam, Cambodia and Laos was to check China's rising influence in Southeast Asia, saying it was part of broadening the overall relationship.
"It doesn't really signify more than that," said a senior State Department official, speaking on condition of anonymity.
China is rapidly building up its military and could challenge traditional US naval dominance in the region, experts say, citing among other examples, Beijing's setting up of a new underground nuclear submarine base on the southern tip of Hainan Island, close to vital sea lanes in Southeast Asia.
Unlike many other Southeast Asian states which have substantial military ties with the United States, "we havent had that so much with Vietnam, Laos and Cambodia because of the history," the State Department official said, referring to the Vietnam War.
As the conflict escalated between the United States and Vietnam, neighbors Laos and Cambodia became increasingly involved in the war.
The Ho Chi Minh Trail -- a supply route from North to South Vietnam that the United States wanted to cut -- passed through both countries.
US officials said they were looking at expanding an international military education and training program in Vietnam, now confined largely to ship visits and a modest English language teching project for Vietnamese military officers.
The former battlefield enemies exchanged defense military attaches in the mid 1990's after Hanoi cooperated in accounting for missing Americans from the Vietnam War.
US defense ties in Laos also centered on the recovery of soldiers missing.
China is fast emerging as a top economic player in Cambodia and Laos.
In Cambodia, where the 1975-1979 communist Khmer Rouge regime was backed by Beijing, China is the largest foreign donor.
"It is a fact that China is growing economically and playing a more active role in much of the world, certainly including in Southeast Asia, but we don't see this as a zero sum game," Marciel said.