HAI PHONG, VIETNAM — The US will provide $18 million to Vietnam to help procure coast guard patrol vessels, a first step in what Secretary of Defense Ash Carter hopes is a growing military relationship between those two countries.

Carter's comments came shortly after he became the first US Secretary of Defense to board a Vietnamese military vessel, a Coast Guard ship in the port of Hai Phong.

"We need to modernize our partnerships," Carter said. "After 20 years there is more we can do… [to] enhance our capabilities and the capabilities of Vietnam."

The ships in question are from Louisiana-based Metal Shark, which provides a number of patrol boats to the US Coast Guard.

The move continues a theme of Carter's swing through Asia, which has focused heavily on building up maritime security capacity amongst partner nations concerned over rising Chinese aggression in the South China Sea.

The US Embassy in Vietnam held an industry day in April, with a number of American defense firms present, in hopes that it might spur interest from a Vietnamese military that is increasing ties with the Pentagon.

However, a wave of US sales in the near future is unlikely, in part because of two big factors.

First, as much as Vietnam is on the rise, it remains far behind Asian economic powerhouses like China, Japan or South Korea. Hence, it has to be choosier with its equipment, and is more reliant on transfer of used gear versus buying high-end new technology.

There is also a cultural barrier. The core of Vietnam's military training and equipment is Russian, and transferring toward American goods can be tricky. A US official pointed to simple roadblocks, such as Vietnamese officers being trained to understand instruction manuals from Russian gear struggling with American pamphlets, as situations which can have an outsize impact on military procurement decisions.

The official also noted the government has not clearly stated what it would like to buy from the US, instead talking nebulously about maritime security assets and radar systems.

Potentially helping that situation along is Sen. John McCain, the chairman of the Senate Armed Services Committee, who plans this week to introduce legislation to alter the long-standing ban on lethal weapon sales to Vietnam.

Carted indicated that the Pentagon has consulted with McCain on the issue, and expressed confidence the change would benefit both nations, although he declined to elaborate on what specifically the US could move toward Vietnam.

As to what differentiates the planned McCain legislation from the Obama administration's executive order in October, which eased the way for sales of lethal weaponry for maritime security purposes, Carter framed it as the next step to a broader military relationship.

"What we now have is some authority to assist Vietnam with equipment and technology in the maritime security space, and that's terrific," Carter said. "But we'd obviously benefit from more latitude."

Twitter: @AaronMehta