WASHINGTON (AFP) – A US official voiced concern Wednesday about tensions between China and Vietnam over the resource-rich South China Sea and pledged to defend US oil companies operating in the region.

But State Department official Scot Marciel said the United States would not take sides on the myriad island disputes involving China and its neighbors including Japan, the Philippines and Vietnam.

Testifying before Congress, Marciel said that Beijing has told US and other foreign oil companies to halt work with Vietnamese partners in the South China Sea or face consequences in their business dealings in lucrative China.

"We object to any effort to intimidate US companies," Marciel, a deputy assistant secretary of state handling Asia, told the Senate Foreign Relations Committee.

"We have raised our concerns with China directly. Sovereignty disputes between nations should not be addressed by attempting to pressure companies that are not party to the dispute," he said.

"We have also urged that all claimants exercise restraint and avoid aggressive actions to resolve competing claims," he said.

China, which has historic tensions with Vietnam, has administered the Paracel islands since 1974 when it overran a South Vietnamese outpost shortly before the end of the Vietnam War.

The islands -- known as the Xisha by China -- are considered strategic outposts with potentially vast oil and gas reserves, and rich fishing grounds.

Tension mounted earlier this year when Vietnam named a "president" for a government body overseeing the disputed archipelago.

"US policy continues to be that we do not take sides on the competing legal claims over territorial sovereignty in the South China Sea," Marciel said.

China disputes the Diaoyu, or Senkaku, chain with Japan along with Taiwan, which Beijing also claims as a whole.

Separately, the potentially oil-rich Spratly island chain is claimed entirely or in part by Brunei, China, Malaysia, the Philippines, Taiwan and Vietnam.