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 on Thursday, November 17, Reuters reports.

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

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 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 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 in the South China Sea and stepped up defense cooperation with Vietnam in response, said it was aware of the reports that Hanoi had upgraded some of its facilities on 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.