China on Friday for the first time landed several combat aircraft -- including the long-range, nuclear strike-capable H-6K -- at an island airfield in the sea, triggering international concern.

The move prompted immediate criticism from the US, with a Pentagon spokesman condemning China's "continued militarisation of disputed features in the South China Sea".

But China rejected concerns that the deployment had raised tensions in a region home to vital global shipping routes.

"The South China Sea islands are Chinese territories," foreign ministry spokesperson Lu Kang told a regular briefing.

Lu said moving the bombers into the area was "part of the normal training for the Chinese military", and that the US "sending its own warships and planes to the region... poses a danger to other countries".

Friday's takeoff and landing drills took place on Woody Island, according to Washington think-tank the Centre for Strategic and International Studies.

The island is home to China's largest base in the Paracel Islands, which are also claimed by Vietnam and Taiwan.

Hanoi slammed the drills and demanded Beijing "immediately cease" its military moves in the area, the foreign ministry said in a statement to AFP Monday.

The exercises "seriously violated Vietnam's sovereignty... raising tensions (and) destabilising the region", it said.

Vietnam has long traded barbs with its communist neighbour over competing claims in the sea but tensions have risen in recent weeks.

Earlier this month Hanoi told Beijing to remove military equipment from the Spratly islands after CNBC reported it had installed anti-ship and surface-to-air missiles.

Soon after, the Vietnamese unit of Russian oil firm Rosneft said it started drilling in a part of the sea also claimed by China. Beijing responded by asserting its sovereignty over the area.

China claims almost all the South China Sea and has built a string of artificial islands in contested areas since 2013, installing an array of airbases, radar systems and naval facilities.

The Philippines, Taiwan, Brunei and Malaysia have competing partial claims.

The Philippines, which has largely backed off from the sea dispute under China-friendly president Rodrigo Duterte, said Monday said it was taking "appropriate diplomatic action" over the bomber exercises.