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 US think-tank reported on Thursday (Nov 17).

Satellite images taken this month showed Vietnam had lengthened its runway on Spratly Island from less than 760m to more than 1km, Washington's Asia Maritime Transparency Initiative said.

AMTI, a project of the Centre for Strategic and International Studies think-tank, said continued reclamation work would likely mean the runway was extended to more than 1.2km.

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 23ha 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.

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.