US President Joe Biden is sending a clear message to Beijing that Washington is bolstering its alliances in the Indo-Pacific in the face of China's growing assertiveness in the region, according to London's Financial Times.

At the first meeting, with Japanese Prime Minister Fumio Kishida the two leaders will announce a plan to implement the most significant upgrade to the alliance since the nations signed a mutual defence treaty in 1960, said the FT.

Then he meets President Ferdinand Marcos Jr for the first US-Japan-Philippines trilateral summit.

The meetings come amid the allies' mounting alarm at China's actions in the region, from military activity around Taiwan to incursions in waters near the Senkaku Islands in the East China Sea that are administered by Japan but claimed by China.

A source of deepening concern is the Second Thomas Shoal, a reef in the Spratly Islands inside the Philippines' exclusive economic zone.

Chinese coastguard ships have used water cannons and other aggressive measures to stop the Philippines from supplying marines stationed on the Sierra Madre, a rusting ship that has been lodged on the reef for 25 years.

Manila ran the vessel aground in 1999 to bolster its claim to the contested reef and routinely sends boats to resupply the marines. China accuses the Philippines of bringing construction materials to reinforce the ship.

China claims sovereignty over most of the South China Sea and is taking aggressive action towards the Philippines at the shoal. It accuses Manila of reneging on what it says was a deal years ago to remove the ship from the reef.

Some US experts believe Beijing wants to remove the Sierra Madre and build a military outpost - as it has done elsewhere in the South China Sea - to provide a strategic foothold close to American forces deployed in the Philippines for any war over Taiwan involving the US and China.

Admiral John Aquilino, head of US Indo-Pacific Command, said he was "very, very concerned" about "dangerous and illegal" Chinese actions around the reef.

The US and its allies - Japan, Australia and the Philippines - have held their first joint military exercises inside the Philippines' EEZ, in an effort to boost deterrence against China, said Admiral Aquilino.

US officials recently told the FT that Biden would use appearances at the summits to warn China that the mutual defence treaty between the US and the Philippines extended to the Sierra Madre.