The United States has offered $32.5m in assistance to Southeast Asian nations, more than half to Vietnam, to boost maritime security, which comes as tension grows with China over rival claims in the South China Sea.

On his first visit to Vietnam as secretary of state, John Kerry denied on Monday that the assistance had anything to do with China.

He did, however, call for "intensified negotiations and diplomatic initiatives" between China and Japan on resolving differences in the East China Sea.

This is part of a gradual and deliberate expansion that has been planned for some period of time which we have been working on ... an ongoing policy and not some kind of quickly-conceived reaction John Kerry, US secretary of state

Kerry said up to $18m of the funds would go towards strengthening Vietnam's coastal patrols to help its coastguard react quicker to search and rescue missions, and for disasters.

The funding would also be used to buy five "fast" patrol boats for Vietnam's coastguard in 2014, he added.

"This announcement has nothing to do with a recent announcement by any other country," Kerry told a joint news conference with his Vietnamese counterpart, Pham Binh Minh.

"This is part of a gradual and deliberate expansion that has been planned for some period of time which we have been working on," he said, adding that "this is really an ongoing policy and not some kind of quickly-conceived reaction" to increased tension.

Still, Kerry said the US opposed "coercion and aggressive tactics" to advance territorial claims, saying any disputes should be resolved through international institutions.

Territorial claims

China has claimed almost the entire oil- and gas-rich South China Sea, overlapping in different places with claims made by Taiwan, Malaysia, Brunei, the Philippines and Vietnam.

The US has said it is neutral in the dispute, centered on China's historic claim of waters deep in the maritime heart of Southeast Asia, but is determined to preserve peace and ensure that sea-lanes vital for the world economy are not hindered.

China also has disputes with Japan and South Korea over different sets of tiny islands in the East China Sea.