The United States and the Philippines have agreed on five locations for US military bases in the Philippines under a security agreement inked amid rising tensions with China in the South China Sea, US officials said on Friday.

The US State Department named the five as Antonio Bautista Air Base, Basa Air Base, Fort Magsaysay, Lumbia Air Base, and Mactan-Benito Ebuen Air Base.

US Deputy Assistant Secretary of Defence Amy Searight said the deal was reached under the Enhanced Defence Cooperation Agreement (EDCA) signed in 2015 that grants Washington increased military presence in its former colony through rotation of ships and planes for humanitarian and maritime security operations.

Searight told the opening of the annual US-Philippines Bilateral Strategic Dialogue in Washington that Manila was a "critical US ally" and ties had never been stronger. US Defence Secretary Ash Carter is set to visit the Philippines in April.

Searight also said the Pentagon had told the US Congress of its intention to provide $50 million to help build maritime security in the region and that the Philippines would get "the lion's share."

The funds are expected to go towards improving radar and other monitoring capabilities in the South China Sea (SCS), where China's pursuit of territorial claims has raised US concerns and those of rival claimants, including the Philippines.

In January, the Philippines said it had offered eights bases for US use, including the former US air force base of Clark and the former US Navy base at Subic Bay, and two sites on Palawan island near the South China Sea. Philippines Defence Undersecretary Pio Lorenzo Batino said Manila was pleased with the finalisation of the locations.

US Assistant Secretary of State Daniel Russel said the deal would speed U.S. help in response to natural disasters and facilitate modernization of the Philippines armed forces. He said it came at an important time ahead of a ruling in a case the Philippines has brought against China over its South China Sea claims in the International Court of Arbitration in the Hague.

On Thursday, the US Navy said it had seen activity around a reef China seized from the Philippines nearly four years ago that could be a precursor to more Chinese land reclamation in the South China Sea.

Admiral John Richardson also expressed concern that the Hague ruling, which is expected in late May, could prompt Beijing to declare an exclusion zone in what is one of the world's busiest trade routes.