MANILA (BLOOMBERG) - The United States has sent a fighter jet-carrying warship to join drills near the disputed Scarborough Shoal for the first time, sending a pointed message to China as tensions simmer over territorial claims in the region.

The USS Wasp - an amphibious assault ship outfitted last year with F-35B jets - joined the annual Exercise Balikatan with the Philippines this month.

A ship matching the USS Wasp's description was spotted in waters "near the Scarborough Shoal", a feature occupied by China since a tense stance off in seven years ago, the Philippines' ABS-CBN News reported on Tuesday (April 9).

The USS Wasp did not pass within 25 nautical miles of the shoal, according to a US defence official who spoke on the condition of anonymity.

The figure suggests that the ship was not conducting a so-called freedom of navigation operation, a practice criticised by China that the US uses to assert international sailing rights within 12 nautical miles of disputed features.

FREE AND OPEN

The moves come as officials in Manila escalate protests over the presence of more than 200 Chinese ships near the Philippine-occupied South China Sea feature of Thitu.

Philippine President Rodrigo Duterte has urged China to "lay off" the feature, saying he would order a military action.

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The US has been seeking to firm up its treaty alliance with the strategically located South-east Asian nation ever since Mr Duterte was elected in 2016 and announced his "separation" from Washington in favor of Beijing.

In March, US Secretary of State Michael Pompeo assured local officials during a visit to Manila that their treaty would apply to Philippine vessels or planes attacked in disputed waters.

Philippine Foreign Affairs Secretary Teodoro Locsin said earlier this week that the US would remain the country's only military ally.

More than 7,000 soldiers from the Philippines and the US are participating in Exercise Balikatan, according to General Benjamin Madrigal Jr, the Philippine military chief.

The combined information bureau for the joint military drills said on Tuesday that the USS Wasp "has been training with Philippine Navy ships in Subic Bay and international waters of the South China Sea as part of the Exercise Balikatan for several days".

The training is focused on "maritime security and amphibious capabilities, as well as multinational interoperability through military exchanges", the bureau said.

This year's joint exercise was the first to incorporate the USS Wasp with the US Marines Corps' F-35B Lighting II aircraft.

"Together, they represent an increase in military capability committed to a free and open Indo-Pacific," the bureau said. "Participating in Balikatan demonstrates their ability to forward deploy in support of an ally should a crisis or natural disaster occur."