HAI PHONG, Vietnam (REUTERS) - US Defense Secretary Ash Carter pledged $18 million (S$24.2 million) on Sunday to help Vietnam buy US patrol boats, shortly after touring a Vietnamese coast guard vessel that was hit by a Chinese ship during a skirmish in the South China Sea.

Dr Carter, who has been focusing on maritime security during an 11-day trip to the Asia-Pacific, visited the Vietnamese navy headquarters and coast guard headquarters before boarding the ship.

Dr Carter is on his second visit to Asia since becoming defense secretary earlier this year. He said he planned to sign a"vision statement" on Monday with his Vietnamese counterpart to guide the expansion of bilateral military ties.

"We need to modernise our partnership," he told reporters during a visit to the northern city of Hai Phong.

"After 20 years, there is more we could do together."

As part of that effort, he said Washington would provide US$18 million to help Vietnam buy US-made Metal Shark patrol boats to help Hanoi improve its maritime defence capabilities.

US and Vietnamese navies had been "building habits of cooperation" by working together, he said. In April they conducted exercises under rules laid out in the Code for Unplanned Encounters at Sea, a set of regulations approved by multiple maritime nations last year at a meeting in China.

Dr Carter opened his trip to the Asia-Pacific by calling for all countries to stop island-building efforts in disputed, resource-rich regions of the South China Sea. He reiterated that call at the Shangri-La Dialogue security conference on Saturday.

Dr Carter recognised that several countries, including Vietnam, had conducted land reclamation projects in the region but said Chinese activity, covering some 2,000 acres, had outstripped the others and raised questions about Beijing's long-term intentions.

"I will be discussing further with Vietnam the proposal that I was talking about in Singapore, namely for all of the claimants to these disputed areas of the South China Sea ... permanently to halt reclamations," he said.

The Vietnamese coast guard vessel toured by Dr Carter was targeted with Chinese water cannons and rammed by a Chinese ship during a confrontation last year.

The incident occurred after China moved an oil drilling rig into waters claimed by Vietnam. The rig remained for about two months protected by Chinese navy vessels, which were repeatedly challenged by Vietnamese ships.