Security issues in the disputed South China Sea helped convince the Philippines to delay quitting a key US military pact, the nation's envoy to Washington said yesterday.

President Rodrigo Duterte announced Tuesday that the country had suspended plans to cancel the Visiting Forces Agreement, a deal that is important to Washington's moves to counter Beijing's rising regional power.

Duterte has cozied up to China in search of trade and investment, sparking US concern that its long-time ally and former colony would change sides in a strategic boost to Beijing.

"Because of security issues ... in that part of the world [South China Sea], both our governments have seen it would be prudent for us to simply suspend any implementation of the termination," Philippine Ambassador to the US Jose Manuel Romualdez said.

Since taking power in 2016 Duterte has moved closer to Beijing, but has faced pushback from the public and military that is wary of its territorial ambitions in the South China Sea.

Philippine analyst Richard Heydarian said the VFA reversal showed Duterte had to decide between an aggressive China and a helpful historic ally. "This is not the time to initiate an ugly divorce, especially when China is spreading its tentacles everywhere," he said.

The 1998 pact is key to the US-Philippines' broader decades-old military alliance and underpins joint military activities, speedy disaster aid and anti-terror efforts.

Manila's termination of the pact was to have taken effect in August, triggered by the cancellation of the visa of Ronald Dela Rosa, a senator who oversaw Duterte's drug war.

The suspension is for six months from June 1 and can be extended a further six months, which would push the deal to the final year of Duterte's constitutionally-mandated single term.

Philippine and US Marines take part in a beach assault exercise facing the South China Sea. AFP

Sea row prompts manila U-turn on ending US pact