US Defense Secretary Ashton Carter discussed halting land reclamation in the South China Sea with his Vietnamese counterpart in Hanoi on Monday in talks focused on maritime security.

The meeting came after the United States on Saturday called for an immediate end to all such work in the disputed waters where both Vietnam and Beijing have reclaimed land.

"The US and Vietnam are working together to ensure peace and stability in this region and beyond," Carter told reporters at a press conference in Hanoi following "in-depth" talks on regional maritime disputes with Vietnamese Defence Minister Phung Quang Thanh.

Thanh did not reveal Hanoi's position on the US call to halt reclamation but insisted its activities were not an attempt at expansion.

"We have some activities to enhance and consolidate the islands that are under our sovereignty," Thanh told reporters. "We do not expand the islands, we just consolidate to prevent erosion because of waves."

He added that Vietnam has soldiers stationed on its 21 islands and reefs.

On Saturday at a high-level defence dialogue in Singapore, Carter called for "an immediate and lasting halt to land reclamation by all claimants," in the South China Sea, adding the US opposed "any further militarisation of disputed features".

Vietnam has previously admitted carrying out reclamation on islands in the disputed waters but the scale of the work is dwarfed by that of Beijing.

China claims nearly all of the South China Sea and is locked in territorial disputes with Brunei, Malaysia, the Philippines and Taiwan as well as Vietnam.

Carter also pledged $18 million on Monday to help Vietnam buy US-made Metal Shark patrol boats, hailing progress in a bilateral relationship that marks two decades this year since the US and Vietnam normalised ties in 1995.

Their relations had previously been strained by decades of animosity after the Vietnam war, which ended in 1975.

Carter, in Vietnam as part of a 11-day trip to Asia, visited a navy headquarters and toured a coast guard vessel on Sunday,

The former wartime foes also discussed cooperation to ease the legacy of war in Vietnam on Monday, including cleaning up Agent Orange dioxin residue.