Amid the ongoing South China Sea row, Vietnam is extending a runway on an island it claims, a U.S. think tank reported Thursday. The move is seen as an apparent response to China's construction of military facilities on artificial islands in the region.

Washington's Asia Maritime Transparency Initiative said that satellite images from this month showed Vietnam lengthened its runway on Spratly Island from less than 2,500 feet to more than 3,300 feet. According to AMTI, which is a project of the Center for Strategic and International Studies think tank, continued reclamation work by Vietnam would likely mean that the runway has been extended to more than 4,000 feet.

Maritime surveillance aircraft and transport planes as well as combat aircraft can be accommodated in the upgraded runway, Reuters reported, citing AMTI. The report also said that Vietnam had added about 57 acres of land to Spratly Island in recent years.

China, which claims most of the South China Sea, has built military-length runways on three artificial islands since 2013. 

Beijing's reclamation work in the region has been heavily criticized by the United States. Washington said it was aware of reports that Vietnam 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," Anna Richey-Allen, a spokeswoman for the U.S. State Department, reportedly said.

China has blamed the U.S. for heightening tensions in the South China Sea, through which over $5 trillion of maritime trade passes annually. Brunei, Malaysia, the Philippines and Taiwan also have conflicting claims to the waters.