Philippine National Security Adviser Eduardo Ano "expressed his appreciation for the United States' continued assurances and reaffirmation of its ironclad commitment" to their alliance, the Philippine National Security Council said in a statement.

The phone call on Monday was on the heels of a series of maritime run-ins and heated verbal exchanges between China and the Philippines that has triggered concern about an escalation at sea.

President Ferdinand Marcos Jr said last week there would be "countermeasures" against aggression by China's coastguard, while Beijing accused the Philippines of treachery and reneging on a promise to tow away an old naval vessel grounded intentionally on a disputed shoal. Manila denies ever making that pledge.

Philippine Defense Secretary Gilberto Teodoro told Filipinos in an open letter on Tuesday to "not fall into the trap set by Chinese propaganda".

China claims almost the entire South China Sea as its territory, policed by an armada of coastguard vessels, some more than 1,000 km away from its mainland. China has maintained its responses have been appropriate in the face of Philippine encroachment.

The rows come at a time when the Philippines and United States are deepening military ties, frustrating China, which sees Washington as interfering in its back yard.

The Philippines has insisted it never agreed to tow away the BRP Sierra Madre, which has been guarded by a handful of soldiers since it was grounded at the Second Thomas Shoal 25 years ago. China has been accused of blocking resupply missions to those troops.

The former spokesperson of Rodrigo Duterte, the previous president, has confirmed there had been an informal "gentleman's agreement" with China to keep the status quo at the shoal, but not to tow the ship away.

NSC spokesperson Jonathan Malaya said the Marcos government had not seen any document to support China's claim of a Philippine promise to remove the ship.