(The opinions expressed here are those of the author, a columnist for Reuters)

LONDON, April 12 (Reuters) - In early December, a flotilla of two dozen Chinese fishing boats and escort warships sailed to here the disputed Filipino-occupied reef of Thitu. By the end of the month, Beijing had almost 100 vessels in and around the archipelago, sparking an initially largely hidden confrontation that could yet spark outright war.

When China’s People’s Liberation Army Navy turns 70 in October, the celebrations will centrehere its largest, highest profile warships – particularly its pair of aircraft carriers, set to be the first of many. High-tech sabre-rattling is clearly at the heart of Beijing’s strategy to dominate its immediate neighbourhood, with jets and warships particularly aggressive around Taiwan’s borders in the last six months.

What the confrontation with the Philippines demonstrates, however, is that such conventional naval posturing is complemented by something much less conventional: the hundreds if not thousands of small fishing and other vessels of China’s “maritime militia”. Usually unarmed, albeit increasingly escorted by Chinese warships and Coast Guard cutters, they have become more assertive by the month.

With U.S.-Chinese relations already complicated by President Donald Trump’s trade war, such confrontations are now clearly drawing in the United States. Last week, the U.S. Navy announced it was here sending the assault ship USS Wasp – essentially a small aircraft carrier operating 20 F-35B Joint Strike Fighters and a Marine Expeditionary Force – to exercise with the Filipino Navy. On March 1, U.S. Secretary of State Mike Pompeo warned any Chinese attack on Filipino forces or civilian vessels would trigger a U.S. military response under a 1951 mutual defence treaty.

The current confrontation with the Philippines suggests Beijing’s face-off with its neighbours is reaching a new and potentially more volatile stage. China has spent the intervening time building a network of sometimes vast reclaimed outposts on some of the most contentious islands. Now, however, Beijing is becoming much more assertive right up to territory held by other nations, both with conventional military force and “civilian” vessels such as its fishing fleet.

This has also demonstrated how complex the political dynamics of such a face-off can be. Filipino President Rodrigo Duterte had been considered here one of the closest regional leaders to China, with Beijing much less likely than the United States to criticize his government’s often brutal crackdown on suspected drug dealers that has left thousands dead. Chinese investment in the Philippines has increased sharply under his administration, including purchases of major strategic commercial port infrastructure.

Throughout this most recent crisis, Duterte has walked an awkward path between placating hawkish voices at home and further antagonizing his unravelling relationship with Beijing. Nationalist Filipino voices, meanwhile – as well as much of the country’s national security establishment – have been pushing him relentlessly towards a tougher line.

The current face-off began when the Philippines stepped up construction of its own island military outpost, albeit on a much smaller level than China’s giant island-building elsewhere in the region. With over half the world’s fishing fleet, Beijing has more than enough vessels to swarm an area.

Satellite footage here from the Asia Maritime Transparency Initiative, part of the Center for Strategic and international Studies in Washington DC, shows the flotilla almost always supported by one or more Chinese warships or Coast Guard vessels, usually keeping their distance several miles further from any of the disputed islands. Their message appears clear – that if the Philippines chose to follow Indonesia or Vietnam in firing warning shots at Chinese fishing vessels or seizing them in disputed waters, they would be risking an immediate military clash.

How far either side is truly willing to go remains extremely unclear. While Duterte has said war with China would be “suicidal”, he’s also warned here he would send the Filipino military to confront China if it did not “lay off” its islands. Still, it remains likely Beijing will maintain the status quo, frequently or continually maintaining large flotillas around both Thitu, scene of the current dispute, as well as Scarborough Shoal, where a similar face-off took place in 2012.

Even that, it is increasingly clear, would bring with it a significant rise in regional tension. While the Pentagon has remained largely tight-lipped about the exact location and activities of the “Wasp” and her task force, Filipino media cited reports from here&IR=T local fisherman putting her near Scarborough Shoal, also known for regular Chinese fishing fleet and possible “maritime militia” activity. If that were true, that would be the most assertive the U.S. Navy has been in the region when it comes to pushing back at China’s maritime claims.

To make matters more complex still, a small flotilla of Russian warships is now also here exercising in the South China Sea, the latest sign of growing cooperation between Moscow and Beijing. That included an apparently prearranged visit to the Philippines, another sign of just how conflicted Duterte and others in his government remain on their alliance to the United States – and keen to hedge their bets.

In the almost a decade since the Obama administration announced its “pivot” to confront a rising China, both Washington and Beijing have invested considerable energy in imagining and preparing for conventional all-out war with each other. Such a conflict would prove devastating to the economies of both countries and the wider world – a key reason it has not happened so far.

Through pushing its neighbours in slightly less direct military ways, China clearly hopes to dominate the region without needing to fire a shot. But we may have decades more of these confrontations to come – and with other nations increasingly pushing back, they will become riskier by the year. *** Peter Apps is a writer on international affairs, globalisation, conflict and other issues. He is the founder and executive director of the Project for Study of the 21st Century; PS21, a non-national, non-partisan, non-ideological think tank. Paralysed by a war-zone car smash in 2006, he also blogs about his disability and other topics. He was previously a reporter for Reuters and continues to be paid by Thomson Reuters. Since 2016, he has been a member of the British Army Reserve and the UK Labour Party, and is an active fundraiser for the party. (Editing by Giles Elgood)