China’s rapid construction of a string of artificial islands in the South China Sea has entered a more troubling phase. According to a major Pentagon report released last week, Beijing is shifting its focus from land reclamation to building deepwater ports, military-grade airstrips and other strategic infrastructure on top of its islands.

While it’s unclear whether military forces will also be deployed, these sophisticated outposts will enable Beijing to bolster its power projection presence throughout the South China Sea. The United States is now struggling to devise a strategy to prevent China from militarising its newly completed islands. A new level of collective diplomatic pressure is required.

To date, Washington’s reliance on high-profile warnings has failed to deter Beijing. China has blatantly ignored demands by secretary of state John Kerry and secretary of defence Ashton Carter for all nations to stop building in the disputed Spratly and Paracel archipelagos. Beijing has also shrugged off veiled threats from Washington that it won’t tolerate restrictions on freedom of navigation or overflight around the facilities, even as Chinese military officers use radio signals to caution foreign navies and coastguards against getting too close.

Beijing calculates that Washington will not risk a confrontation over the tiny islands, thus it does not take America’s threats very seriously. The US has made clear that it won’t accept China’s militarisation of the South China Sea, but it’s short on options to stop Beijing without triggering a conflict. And while the US Navy has spearheaded calls to sail warships within 12 miles of the islands on freedom of navigation operations, even this won’t ensure that China’s outposts remain peaceful.

In fact, such actions may provoke Beijing to step up the deployment of military forces, raising the spectre of a more contested and volatile strategic environment. The key to dissuading China from militarising its outposts is to identify stakes that are important to Beijing and impose credible costs in these areas in a coordinated way.

Targeting China’s international reputation is a good place to start. Despite popular misconceptions, Beijing is actually extremely concerned about maintaining a positive image. It knows that being viewed as a pariah could bolster international moves to counter its strategic, political and economic weight. Aside from complicating its ascendency in Asia, this would undermine China’s lucrative relations with Europe, the Americas and many international organisations. Diplomatic efforts that paint China as a rule-breaker may thus have some impact in shaping its behaviour.

Right now, the US is conspicuously alone in directly criticising China’s actions in the South China Sea. Respected global bodies – like the European Union or the Group of Seven – and coalitions of responsible regional players – like Australia, New Zealand and possibly Singapore – should also explicitly call out China’s island militarisation as illegitimate and destabilising. This would signal that a threshold had been crossed in the world’s tolerance of China. Moreover, their neutrality and widespread international respect would inject a new level of legitimacy into US-led criticism, making it harder for Beijing to dodge the reputational fallout.

Public diplomacy initiatives that shame China’s conduct also have an important role to play. But they require greater regional solidarity to be truly effective. For instance, the publication of satellite images that track the scale of China’s man-made islands and the use of surveillance flights to record its attempt to cordon off maritime no-go zones have successfully put Beijing’s provocative actions in the spotlight. Yet they’ve failed to effectively isolate China due to inconsistencies among America’s regional allies and partners.

Despite calling on Beijing to stop its activities, the Philippines, Vietnam and Taiwan are all actively renovating their own island facilities, albeit on a vastly smaller scale. Meanwhile, India, Malaysia, Vietnam and Thailand endorse Beijing’s legal position that foreign military activities can be blocked throughout the entirety of a country’s 200-mile exclusive economic zone, rather than just in the 12-mile territorial sea. This disunity has enabled Beijing to shrug off allegations that its South China Sea actions are out of step with the region. All this makes it harder to forge a broader global consensus on China’s destabilising behaviour.

To strengthen the diplomatic campaign against Beijing’s South China Sea outposts, the US and its Asian allies and partners need to align their views on what counts as unacceptable maritime conduct. Washington should ensure that its Asian partners stop their own building activities and bring their views on exclusive economic zone rights in line with the dominant international position.

Finally, if more robust diplomatic measures prove to be necessary, Washington and its regional allies and partners could consider complicating Beijing’s ambitious “One Road, One Belt” New Silk Road initiative. In light of the rapid slowdown in China’s industrial sector, Beijing is trying to speed up efforts to roll out this plan, which aims to create a network of railroads, highways, pipelines, and port facilities stretching from Europe and Central Asia to Africa, the subcontinent and south-east Asia. While this initiative is vital for China’s domestic development in a post-boom economy, it relies on the dozens of Indo-Pacific countries that must agree to Chinese investment and infrastructure projects.

A coordinated push to link cooperation on the New Silk Road to China’s behaviour in the South China Sea is likely to get the attention of policymakers in Beijing. While only a small group nations would agree to this tactic, a concerted effort by India, Sri Lanka, several European countries and America’s south-east Asian partners would significantly complicate Beijing’s trade objectives.

To be very clear, there is nothing inherently worrying about Beijing’s “One Road, One Belt” strategy. If realised, it will inject much needed capital and infrastructure throughout the region. Yet, insofar as it represents a set of economic opportunities that are more critical to China than to other regional players, it may serve as a useful negotiating chip to deter Beijing from pushing ahead with militarising its South China Sea outposts.

These diplomatic measures offer a way to ratchet up international pressure on China without resorting to more muscular policies. As Beijing tends to respond assertively to acts of perceived foreign aggression, it is likely to become more determined about militarising its South China Sea islands if US warships launch high-profile missions within 12-miles of its outposts. By contrast, increasing China’s sense of international isolation and imposing subtle costs on its economic and political interests may be a more prudent and effective policy response.