MANILA (Reuters) - The Philippines has agreed to allow the United States access to its military bases under a new security deal being negotiated by the two allies, amid mounting concern over China's increasing assertiveness in the disputed waters of the South China Sea.

The offer was made during a sixth round of talks held in Washington last week, Filipino officials said on Friday. The two sides hope to finalize terms before U.S. President Barack Obama embarks on a visit to Asia, including the Philippines, next month.

"Consensus was arrived at on many provisions of the draft agreement," Pio Lorenzo Batino, defense undersecretary told a news conference, adding the deal is 80 percent done.

"The proposed agreement will allow the sharing of defined areas within certain AFP (Armed Forces of the Philippines)facilities with elements of the U.S. military."

The United States plans to "rebalance" its forces in Asia-Pacific region, and has similar arrangements with Australia and Singapore, as part of its strategy to counter China.

The new agreement on enhanced defense cooperation will allow the United States increased deployment of troops, ships, aircraft and humanitarian equipment.

U.S. military access in the Philippines is currently limited to during annual joint-exercises and port visits. The Philippines kicked the United States out two large military bases, including Subic Bay, in 1991.

While that ended a special relationship going back 40 years between the United States and its former colony won its independence in 1946, an alliance has endured.

Manila would welcome the return of a U.S. military presence to deter China's ambitions in the South China Sea, and to help provide humanitarian assistance during natural disasters.

"It will not stop China from its bullying tactics, but it will become more cautious and might exercise self-restraint due to the U.S. presence," Rommel Banlaoi, an analyst at Philippine Institute of Peace, Violence and Terrorism Research, said.

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"The Philippines will also benefit from more exercises and more assistance from the U.S. and it will elevate the Philippines to a major non-NATO ally in the Pacific."

Friction between China and the Philippines and other states in the region, over disputed territories in the South China Sea has increased since last year despite diplomatic efforts to forge an agreement on maritime conduct.

The dispute revolves round competing claims over the Spratlys, a group of 250 uninhabitable islets spread over 165,000 square miles.

On Sunday, three Chinese coast guard ships stopped two Filipino civilian vessels from delivering food, water and construction materials to troops based on a ship that was deliberately run aground on reef in the Spratlys in 1999 to reinforce the Philippines' claim.

Manila called the Chinese actions "a clear and urgent threat to the rights and interests of the Philippines".

Under the draft accord, the Philippines will allow U.S. forces joint use of facilities in several military bases like Manila, Clark, Palawan, Cebu, Nueva Ecija, and La Union, a military official with knowledge of the negotiations.

"We are only offering U.S. military forces access to fewer military bases," Ambassador Eduardo Malaya, a member of the panel negotiating with the Americans, said.

Manila refused a request for use of civilian airfields and ports, like Subic Freeport Bay, Laoag and Davao international airports, according to the military official.

Batino said the agreement will be legally binding but will not require ratification by the Philippine Senate, which could delay the actual U.S. deployment.

(Reporting By Manuel Mogato; Editing by Simon Cameron-Moore)