WASHINGTON (Reuters) - Vietnam is extending a runway on an island it claims in the South China Sea in apparent response to China's building of military facilities on artificial islands in the region, a U.S. think tank reported.

Satellite images taken this month showed Vietnam had lengthened its runway on Spratly Island from less than 2,500 feet (760 meters) to more than 3,300 feet (1 km), Washington's Asia Maritime Transparency Initiative said on Thursday.

AMTI, a project of the Center for Strategic and International Studies think tank, said continued reclamation work would likely mean the runway was extended to more than 4,000 feet (1.2 km).

It said the upgraded runway would be able to accommodate maritime surveillance aircraft and transport planes, as well as combat aircraft.

The report said Vietnam had added about 57 acres (23 hectares) of land to Spratly Island in recent years, but its reclamation work remained modest by Chinese standards.

China has built military-length runways on three artificial islands it has built up in the South China Sea since 2013.

The United States, which has criticized China's reclamation work and stepped up defense cooperation with Vietnam in response, said it was aware of the reports that Hanoi had upgraded some of outposts in the Spratly Islands.

"We encourage all claimants to take steps to lower tensions and peacefully resolve differences," said Anna Richey-Allen, a spokeswoman for the U.S. State Department.

Chinese Foreign Ministry spokesman Geng Shuang said China had indisputable sovereignty over the Spratly Islands and that it was "firmly opposed to relevant countries' illegal occupation".

"We again strongly urge relevant countries to respect China's sovereign rights and interests, immediately stop their illegal occupation and illegal construction activities, and remove all people and installations," Geng told a regular press briefing on Friday.

Reuters reported in August that Vietnam had discreetly fortified several of its islands in the disputed South China Sea with mobile rocket launchers capable of striking China's runways and military installations across the vital trade route.

Military analysts said the deployment of the launchers was the most significant defensive move Vietnam has made on its holdings in the South China Sea in decades and it underscored Hanoi's concerns about China's assertive pursuit of territorial claims in the disputed region.

Vietnam's Foreign Ministry has called the information "inaccurate", without elaborating.

(Reporting by David Brunnstrom and the Beijing newsroom; Editing by James Dalgleish and Kim Coghill)