On February 5, 2014, US Assistant Secretary of State for East Asian and Pacific Affairs Daniel Russel said at a congressional hearing that China was "lack of clarity with regard to its South China Sea claims has created uncertainty, insecurity and instability in the region." He also urged China to clarify its nine-dash line claim. This was the first explicit and official comment made by the US to challenge China on the South China Sea issue. And obviously the US was well aware that, as the Nansha Islands dispute was still unsettled, any attempt to clarify the dash line or maritime claims would only lead to an escalation of tensions. In the same month, US Chief of Naval Operations (CNO), Admiral Jonathan Greenert announced US's support for the Philippines in the event of a China-Philippines conflict. This is the toughest stance expressed by the US in the China-Philippine dispute. At the Post Ministerial Conference of the ASEAN Foreign Ministers' Meeting in Naypyidaw in August 2014, US Secretary of State John Kerry directly called for a moratorium on land reclamation, building on disputed islands, and actions that might further escalate disputes.

The US started to opt for a cost-imposing strategy against China, meaning to make it more costly for China to take any actions in the South China Sea by resorting to political, diplomatic, public opinion and military means, so as to force China to pull back without inciting arms confrontation. In 2015, the US released three strategic security documents, titled Forward, Engaged and Ready: A Cooperative Strategy for 21st Century Seapower, National Security Strategy, National Military Strategy and Asia-Pacific Maritime Security Strategy, respectively, all of which talked about the South China Sea issue at fairly great length, and asserted that the US would make China pay the price.

From the Chinese perspective, as well as undermining the US credibility as a potential mediator, the US's dramatically altered policy on the South China Sea has heightened China's fears that its interests would be further undermined, thus inspiring its determination and measures to defend them.

choing its policy readjustments, the US has accelerated provocative and coercive actions that are clearly targeted at China. For example, the US’s surveillance at the Nansha Islands and its surrounding waters have intensified. The number of sorties flown by the US planes to conduct close-in reconnaissance at the South China Sea Islands has increased from about 260 in 2009 to over 1,200 in 2014. Also, as a way to flex its muscle and assert freedom of navigation, the US keeps sending ships to sail within 12 nautical miles of the Nansha Islands or even the non-disputed Xisha Islands. On October 27, 2015, the USS Lassen navigated within 12 nautical miles of Zhubi Reef (Subi Reef). On January 30, 2016, the USS Curtis Wilbur trespassed China’s territorial waters near Zhongjian Island. Quite different from its usual practice, the US media began to buzz over these events. US Pacific Command commander Harry Harris even openly declared to take more sophisticated and wide-ranging activities in the future, and send warships to the South China Sea about twice a quarter.

Other deterrent actions taken by the US include the followings: In July 2015, the new commander of the US Pacific Fleet Admiral Scott Swift joined the surveillance mission on board the ASW P-8A Poseidon to conduct close-in reconnaissance at the South China Sea; on November 5, US Secretary of Defense Ashton Carter cruised on the USS Roosevelt, and when he began to deliver a speech on board, the carrier was churning through the disputed waters about 150-200 nautical miles south of the Nansha Islands and about 70 nautical miles north of Malaysia; on November 8 and 9, two US B-52 strategic bombers flew near the Chinese islands under construction; and during his visit to the Philippines on April 15, 2016, Carter landed aboard the aircraft carrier USS John C. Stennis and joined a patrol in the South China Sea. US warships and planes also frequently conducted "innocent passage" through China’s territorial waters and airspace.

The US has also sought to strengthen its alliance system and forces network surrounding the South China Sea. Since the implementation of the Asia-Pacific rebalance strategy, the US has been stepping up deployment of forces around the South China Sea rim, including the Australian port of Darwin, the Changi Naval Base in Singapore, the Philippines and Malaysia. The US is also enhancing cooperation with Malaysia, Indonesia and Vietnam to conduct intelligence gathering and enhance maritime domain awareness capabilities in the region, and expanding military support to some claimants in the dispute like the Philippines and Vietnam, to help improve their reconnaissance, patrol control and anti-access capacity. In March 2016, the US and the Philippines announced at their sixth annual Bilateral Security Dialogue that the US forces were allowed to use five Philippine military bases. In April 2016, the US and the Philippines conducted again the Shoulder-to-Shoulder exercises in the South China Sea, with more targeted items like retaking over islands, oil rig defense, etc., obviously aiming at disputes in the South China Sea.