The United States has no immediate plans to withdraw a mid-range missile system deployed in the Philippines, despite Chinese demands, and is testing the feasibility of its use in a regional conflict, sources with knowledge of the matter said.

The Typhon system, which can be equipped with cruise missiles capable of striking Chinese targets, was brought in for joint exercises earlier this year, both countries said at the time, but has remained there.

The Southeast Asian archipelago, Taiwan's neighbor to the South, is an important part of U.S. strategy in Asia and would be an indispensable staging point for the military to aid Taipei in the event of a Chinese attack.

China and Russia have condemned the first deployment of the system to the Indo-Pacific, accusing Washington of fueling an arms race.

China's foreign ministry said on Thursday it was very concerned about the plan to keep the system in place.

"It seriously threatens the security of regional countries and intensifies geopolitical confrontation," ministry spokesperson Lin Jian told a press briefing.

The deployment, some details of which have not been previously reported, comes as China and U.S. defense treaty ally the Philippines clash over parts of the hotly contested South China Sea. Recent months have brought a series of sea and air confrontations in the strategic waterway.

Philippine officials said Filipino and U.S. forces continued to train with the missile system, which is on the northern island of Luzon, facing the South China Sea and is close to the Taiwan Strait. They said they were not aware of immediate plans to return it, even though the joint exercises end this month.

A Philippine army spokesman, Colonel Louie Dema-ala, on Wednesday said training was ongoing and it was up to Philippine authorities and the United States Army Pacific to decide how long the missile system would stay.

"It is up to the higher headquarters to decide on its stay, and most importantly the USARPAC because they own this, it is not our capability," he told Reuters.

A public affairs officer for USARPAC said the Philippine army had said the Typhon could stay beyond September and soldiers trained with it as recently as last week, engaging "in discussions over employing the system, with a focus on integrating host nation support."

A senior Philippine government official and another person familiar with the matter said the U.S. and the Philippines were testing the feasibility of using the system there in the event of a conflict and how well it works in that environment. Both spoke on condition of anonymity.

The government official said the Typhon - which is intended to be mobile and moved as needed - was in the Philippines for a "test on the feasibility of deploying it in country so that when the need arises, it could easily be deployed here."

The office of Philippine President Ferdinand Marcos Jr did not respond to a request for comment.

The U.S army flew the Typhon, which can launch missiles including SM-6 missiles and Tomahawks with a range exceeding 1,600 km (994 miles), to the Philippines in April in what it called a "historic first" and a "significant step in our partnership with the Philippines".

A satellite image taken on Wednesday by Planet Labs, a commercial satellite firm, and reviewed by Reuters showed the Typhon at the Laoag International Airport, in Ilocos Norte province.

The senior government official who spoke to Reuters said there were no immediate plans to withdraw it.

"If ever it will be pulled out, it is because the objective has been achieved and it may be brought (back) in after all the repairs or the construction would have been done," the official said, adding that there was strategic value for the Philippines in keeping the system to deter China.

"We want to give them sleepless nights."

The U.S. has been amassing a variety of anti-ship weapons in Asia, as Washington attempts to catch up quickly in an Indo-Pacific missile race in which China has a big lead, Reuters has reported.

Although the U.S. military has declined to say how many will be deployed in the Indo-Pacific region, more than 800 SM-6 missiles are due to be bought in the next five years, according to government documents outlining military purchases. Several thousand Tomahawks are already in U.S. inventories, the documents showed.

China has denounced the deployment of the Typhon several times, including in May when Wu Qian, spokesperson for China's defense ministry, said Manila and Washington had brought "huge risks of war into the region".

Russian President Vladimir Putin in June cited the deployment when announcing his country would resume production of intermediate- and shorter-range nuclear-capable missiles.

Philippine Foreign Affairs Secretary Enrique Manalo in July assured his Chinese counterpart the presence of the missile system in his country posed no threat to China and would not destabilize the region.

China has fully militarized at least three of several islands it built in the South China Sea, which it mostly claims in full despite a 2016 arbitral ruling that backed the Philippines, arming them with anti-ship and anti-aircraft missiles, the U.S. has said.

China says its military facilities in the Spratly Islands are purely defensive, and that it can do what it likes on its territory.