MANILA, March 2 (Reuters) - A decision by the Philippines to grant the United States greater access to its military bases was "not for engaging in war" but meant to enhance its ability to defend itself against external threats, its defence chief said on Thursday.

"The geopolitical situation is becoming more precarious by the day," Carlito Galvez, office-in-charge of the defence ministry, said in a statement.

"We are not preparing for war, rather we are aiming to develop our defence capabilities against eventualities and threats to our security," he added.

His statement came a day after some senators and a provincial governor raised concerns and opposition to President Ferdinand Marcos Jr's decision to give the United States access to four more sites, on top of five locations under a 2014 Enhanced Defense Cooperation Agreement, or EDCA.

EDCA 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 not a permanent presence.

Marcos' decision, announced during last month's visit by U.S. Defense Secretary Lloyd Austin, came amid concern over China's assertiveness in the South China Sea and tension over self-ruled Taiwan.

Galvez has not publicly identified the sites that would be opened to U.S. access.

A former Philippine military chief had said the United States had asked for access to bases in Isabela, Zambales and Cagayan, all on the island of Luzon, facing north towards Taiwan, and on Palawan in the southwest, near the disputed Spratly Islands in the South China Sea.

Cagayan Governor Manuel Mamba has opposed the addition of new sites and told a senate hearing that he did not want EDCA to create problems with China.

"Do not let us tell them that they are our enemies because of the U.S.," said Mamba. "Let them have their own war."

China has said greater U.S. access to Philippine military bases undermined regional stability and raised tensions.

Galvez said EDCA and its defence partnerships "are not intended for aggression."