The Philippines and Vietnam will sign a defence cooperation agreement the office of Philippine President Ferdinand Marcos Jr said, a significant step by two countries that have long opposed China's actions in the South China Sea.

Vietnamese Defence Minister Phan Van Giang was in Manila on Friday to hold talks with his Philippine counterpart, Gilberto Teodoro, and he paid a courtesy call earlier in the day on Marcos.

"We now talk about defence cooperation, security cooperation, maritime cooperation, and certainly, the area of trade as well," Marcos was quoted as saying in a statement that did not specify details of the defence agreement.

"Your visit, I think, will serve as a further impetus, further push to increase that – the depth and the range of our relationship."

The agreement comes at a time of simmering tension in the South China Sea and international concern about an escalation over the conduct of China's vast fleet of coastguards and its activities in the exclusive economic zones of its neighbours.

Balancing act

Vietnam has a tricky balancing act of opposing actions by China that it deems infringements on its sovereignty while needing to maintain close relations with its giant neighbour.

Vietnam's decision to agree comes when US defence allies the Philippines and China are sparring almost every week over the South China Sea, a row that has raged for more than a year.

China claims sovereignty over almost all of the strategic waterway.

Despite their overlapping claims in the Spratly Islands in the South China Sea, where Vietnam and the Philippines occupy atolls and reefs, both countries have expressed a desire to work together and tackle disputes.

Coastguards of the two countries earlier this month held their first-ever joint exercises in Manila, simulating fire-fighting drills and search-and-rescue exercises.

That came after Marcos visited Hanoi in January and signed deals that covered "incident prevention in the South China Sea" and "maritime cooperation".