MANILA, April 10 (Reuters) - Philippine President Ferdinand Marcos Jr assured China on Monday that military bases accessible to the United States would not be used in any offensive action, stressing the arrangement with Washington was designed to boost his country's defences.

China's foreign ministry last week said the United States strengthening military deployment in the Philippines would only lead to more tension in the region, after Manila allowed Washington access to more of its bases.

"China's reaction was not surprising," Marcos told reporters. "The Philippines will not allow the bases to be used in any offensive action," he said.

The Philippines identified last week four more of its bases the United States will get access to, almost doubling the number included in its Enhanced Defense Cooperation Agreement (EDCA).

The locations of the bases are significant, with three facing north towards Taiwan and one near the disputed Spratly Islands in the South China Sea, where China has built artificial islands equipped with runways and missile systems.

EDCA underlines the Philippines' strategic importance to the former colonial ruler the United States, coming at a time of growing concern over China's conduct in the South China Sea and tension over self-ruled Taiwan.

Signed in 2014, it allows U.S. access to Philippine bases for joint training, pre-positioning of equipment and building of facilities such as runways, fuel storage and military housing, but it is not a permanent presence.

"If no one is attacking us, they need not worry because we will not fight them," Marcos said. "What we are doing is we are strengthening our territorial defence, the defence of the Republic."

Marcos' remarks also come ahead of the largest ever joint military exercises between the Philippines and the United States, which will feature for the first time live fire exercises at sea.