Beijing's South China Sea bombers fly in the face of protests

China attracted diplomatic protests after long-range H-6K bombers carried out landing and take-off drills at an unidentified island airstrip on Friday.

But Beijing has long brushed aside such condemnation, denying that it was militarising the region even though it has installed an array of airstrips, radar systems and naval facilities on a string of islands it has reclaimed in contested areas since 2013.

The planes are believed to have landed on Woody Island, China's largest base in the Paracel Islands which is also claimed by Vietnam and Taiwan, according to Bonnie Glaser, a China expert at the Centre for Strategic and International Studies in Washington.

An H-6K long-range strategic bomber deployed to Woody Island could reach almost the entire South China Sea and many countries surrounding it, according to a CSIS analysis.

The steady build-up of military assets in the waterway -- believed to have significant oil and natural gas deposits -- allows China to "influence its weaker neighbours in peacetime", Glaser said.

"Through the use of a large number of law enforcement ships, for example, it can pressure Vietnam and the Philippines not to unilaterally extract energy in waters that China claims jurisdictional rights," she said.

"In wartime, China's military assets on these islands will increase the risk to the US of intervening militarily." Earlier this month China deployed anti-ship cruise missiles and surface-to-air missiles on the disputed Spratly Islands off the Philippine coast, CNBC reported, citing sources close to US intelligence.

Beijing claims virtually all the South China Sea and has ignored partial counter-claims from the Philippines, Vietnam, Taiwan, Brunei and Malaysia.

The Pentagon condemned Friday's bomber drills as a sign of "continued militarisation" of disputed islands while Vietnam said the move violated its sovereignty and raised tensions in the region.

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

Despite the rhetoric, experts say little has been done to prevent China from solidifying its vast maritime claims.

Beijing has managed to weaken regional resistance by courting some members of the 10-nation Association of Southeast Asian Nations.

While US warships have conducted "freedom of navigation" operations near Chinese-claimed features, they have not deterred Beijing.

The bombers were deployed while US President Donald Trump is focused on preparing for a summit with North Korean leader Kim Jong Un next month.

"I think there is an obvious political motivation for the timing," said Euan Graham, director of the international security programme at the Australian think tank the Lowy Institute.

"Because the US is mainly engaged in the North Korea files... there is a window of opportunity where the US reaction is likely to be restrained." China's foreign ministry has repeated denials that the region was being militarised, saying the islands belong to Beijing and the bombers were conducting "normal training".

The latest exercises were "largely symbolic" and not a significant military development, Graham said.

To deploy from Woody Island, China would have to install logistics infrastructure to operate aircraft, refuel them, store weapons and house crews, he said.

"Just landing an aircraft doesn't make it an operational space," Graham said.

It would be more significant if and when China starts flying combat aircraft to the Spratlys, he said, because that would bring northern Australia into missile range.