At the southernmost feature, cargo is unloaded onto the shore of Mischief Reef, located 216 km (135 miles) west of the Philippine island of Palawan, in this Center for Strategic and International Studies (CSIS) Asia Maritime Transparency Initiative satellite image. REUTERS/CSIS'S ASIA MARITIME TRANSPARENCY INITIATIVE/DIGITAL GLOBE/HANDOUT

China's Foreign Ministry has rejected US criticism of construction of artificial islands in disputed areas of the South China Sea. The move, it insists, is meant to protect the country's maritime rights.

Referring to US President Obama's remarks that China is using its size and muscle to push around smaller nations, foreign ministry spokesman Hua Chunying said: "I think it is very clear to everyone who has the greatest size and muscle in the world."

She told reporters that China has always been a firm defender and promoter of peace and stability in the South China Sea, while also claiming that China had "indisputable rights" to the Spratly islands.

The war of words followed satellite images released by the Washington-based think tank Centre for Strategic and International Studies showing Chinese vessels dredging sand onto Mischief Reef and the resulting land spreading in size.

The Philippines reacted strongly, calling for the Asian giant to "dismantle" the reclaimed land.

Beijing asserts sovereignty over almost the whole of the mineral-rich South China Sea, but is opposed by the Philippines, Vietnam, Malaysia, Brunei and Taiwan which have overlapping claims.

The UN Convention on the Law of the Sea does not legally allow for reclaimed land to be used to demarcate 12-nautical-mile territorial zones.

Obama had said the views of smaller claimant nations had to be considered, while pulling up China for not abiding by international norms and rules, but using its "sheer size and muscle to force countries into subordinate positions".

"We think this can be solved diplomatically, but just because the Philippines or Vietnam are not as large as China doesn't mean that they can just be elbowed aside."

The US has been arbitrating the disputed territorial claims, but China has preferred to go by what it calls historical entitlements.

China's dispute with Vietnam led to maritime clashes last summer after China deployed a giant oil rig in waters claimed by both sides.

Beijing has refused to cooperate with an international arbitration case brought by the Philippines regarding territorial rights over the sea.