(Corrects to swap the words 'four' and 'five' in reference to Philippine bases in paragraph 17)
By Trevor Hunnicutt
WASHINGTON, May 1 (Reuters) - President Joe Biden and Philippine President Ferdinand Marcos Jr. will agree on Monday to new guidelines aimed at strengthening military cooperation, said U.S. officials, underscoring a dramatic turnaround in U.S.-Philippine relations over the past year.
The new guidelines focus on military coordination across land, sea, air, space and cyberspace, while the administration will also transfer three C-130 aircraft and look to send additional patrol vessels.
Biden will also reaffirm the countries' 1951 Mutual Defense Treaty, which calls for the United States to act in the event of an armed attack on the Philippine military, said the officials in a briefing for reporters.
The meeting comes as part of a four-day U.S. visit by Marcos that started on Sunday, the first by a Philippine president in more than 10 years.
Marcos, who became president last year, has sought warm relations with both the United States and China, who are vying for influence in the Asia-Pacific region.
Under Rodrigo Duterte, Marcos' predecessor, relations with U.S. soured as he turned the Philippines sharply away from its former colonial ruler and built closer ties with China.
But with many Filipinos frustrated by China's actions in the South China Sea, including the harassing of Philippine ships and fishermen in parts of the sea that both countries claim, popular support has grown for a tougher stance towards Beijing.
Wariness about China's intentions has only increased recently.
A reported comment last month by Beijing's ambassador to Manila that the Philippines should not support Taiwan's independence "if you care about the 150,000 overseas foreign workers" of Filipino origin who live there was viewed as a "veiled threat," a U.S. official said.
"Some of the steps that China have taken have concerned (Marcos), probably even surprised him," said one senior Biden administration official. "He has strong desire to work closely with both countries but finds himself in a situation that the steps that China's taking are deeply concerning."
"We will reaffirm our commitment to fostering our long standing alliance as an instrument of peace and as catalyst of development in the Asia Pacific region," Marcos said before leaving for the trip on Sunday.
PHILIPPINES CENTRAL TO U.S. PLANS
For its part, the U.S. government sees the Philippines as key to any effort to counter an invasion of Taiwan by China, which claims the island as its own territory.