LONDON/BEIJING (REUTERS) - China expressed anger on Thursday (Sept 6) after a British Royal Navy warship sailed close to islands claimed by China in the South China Sea late last month, saying Britain was engaged in "provocation" and that it had lodged a strong complaint.

The HMS Albion, a 22,000-ton amphibious warship carrying a contingent of Royal Marines, passed by the Paracel Islands in recent days, said the sources, who were familiar with the matter but who asked not to be identified.

The Albion was on its way to Ho Chi Minh City, where it docked on Monday following a deployment in and around Japan.

One of the sources said Beijing dispatched a frigate and two helicopters to challenge the British vessel, but both sides remained calm during the encounter.

The other source said the Albion did not enter the territorial seas around any features in the hotly disputed region but demonstrated that Britain does not recognise excessive maritime claims around the Paracel Islands.

Twelve nautical miles is an internationally recognised territorial limit.

The Paracels are occupied entirely by China but also claimed by Vietnam and Taiwan.

China's Foreign Ministry, in a faxed statement sent to Reuters, said the ship had entered Chinese territorial waters around the Paracel Islands on Aug 31 without permission, and the Chinese navy had warned them to leave.

"The relevant actions by the British ship violated Chinese law and relevant international law, and infringed on China's sovereignty. China strongly opposes this and has lodged stern representations with the British side to express strong dissatisfaction," the ministry added.

"China strongly urges the British side to immediately stop such provocative actions, to avoid harming the broader picture of bilateral relations and regional peace and stability," it said.

"China will continue to take all necessary measures to defend its sovereignty and security."

The encounter comes at a delicate time in London-Beijing relations.

Britain has been courting China for a post-Brexit free trade deal, and both countries like to describe how they have a "golden era" in ties.

A spokesman for the Royal Navy said: "HMS Albion exercised her rights for freedom of navigation in full compliance with international law and norms."

British Prime Minister Theresa May's spokesman said Britain had a strong relationship with China.

FREEDOM OF NAVIGATION

China's claims in the South China Sea, through which some US$3 trillion (S$4.1 trillion) of shipborne trade passes each year, are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam. Britain does not have any territorial claims in the area.

While the US Navy has conducted Freedom of Navigation Operations (FONOPs) in the same area in the past, this British challenge to China's growing control of the strategic waterway comes after the United States has said it would like to see more international participation in such actions.

Both Britain and the United States say they conduct FONOP operations throughout the world, including in areas claimed by allies.

The British Navy has previously sailed close to the disputed Spratly Islands, further south in the South China Sea, but not within the 12 nautical mile limit, diplomatic sources have said.

Singapore-based South China Sea expert Ian Storey said Britain had strong traditional interests in defending freedom of navigation but regular deployments in the South China Sea would be constrained due to limited numbers of warships and onerous demands in other parts of the world.

"The UK's actions will please Washington as the Trump administration has grumbled that US allies have been remiss in upholding freedom of navigation in the South China Sea," said Storey, of Singapore's ISEAS Yusof Ishak Institute.

"But China will be displeased as it suggests that US allies are responding to Washington's appeals... It might also nudge other US allies to make similar moves."

FONOPs, which are largely symbolic, have so far not persuaded Beijing to curtail its South China Sea activities, which have included extensive reclamation of reefs and islands and the construction of runways, hangars and missile systems.

Beijing says it is entitled to build on its territories and says the facilities are for civilian use and necessary self-defence purposes. China blames Washington for militarising the region with its freedom of navigation patrols.

Foreign aircraft and vessels in the region are routinely challenged by Chinese naval ships and monitoring stations on the fortified islands, sources have said previously.

In April, warships from Australia - which like Britain is a close US ally - had what Canberra described as a close "encounter" with Chinese naval vessels in the contested sea.