Hours after the United States hosted a historic meeting with Asean leaders that also discussed steps to lower tensions in the South China Sea, reports emerged that China had deployed surface-to-air missiles to a disputed island it controls.
Beijing's move, which analysts said sharply raises the level of militarisation in the region, rattled neighbouring governments as it drew swift condemnation.
Satellite images released by Fox News showed two batteries of missiles and a radar system on Woody Island, part of the Paracel chain also claimed by Taiwan and Vietnam.
Admiral Harry Harris, who heads the US Pacific Command, said the deployment marked a "militarisation of the South China Sea", through which US$5 trillion (S$7 trillion) in ship-borne trade passes every year. Japan, embroiled in a separate territorial dispute with China, questioned Beijing's "unilateral move to change the status quo".
Taiwanese President-elect Tsai Ing-wen said the deployment raised regional tensions. There was no official reaction from Vietnam.
China dismissed suggestions that it was militarising the region. Foreign Minister Wang Yi, who did not confirm or deny the deployment, insisted that Beijing was entitled to "limited and necessary self-defence facilities" under international law.
The tense diplomatic exchange was a far cry from the apparently more restrained discussions on territorial disputes in the South China Sea at the two-day US-Asean Leaders Summit hosted by US President Barack Obama.
An anticipated US warning to China about bullying smaller nations in the region did not materialise. This, despite Vietnamese Premier Nguyen Tan Dung's call on the first day of the summit on Monday for the US to use a "stronger voice and more practical and more efficient actions".
At a post-summit press conference, Mr Obama reiterated US commitment to freedom of navigation and regional order, while saying the leaders discussed "tangible steps" to ease tensions, including a "halt to further reclamation, new construction and militarisation of disputed areas".
A 10-point joint statement issued after the summit also raised eyebrows because it did not explicitly name China.
It was not immediately clear if the summit leaders knew about the missile deployment beforehand or if it was discussed at their meeting.
Speaking to Singapore media after the summit but before the news of the deployment broke, Prime Minister Lee Hsien Loong said the response to the South China Sea issue "is not just verbal statements".
"It is how countries act on the ground, how countries cooperate, negotiate with one another and work out practical solutions which will enable us to get along even though we may have disagreements or conflicting views."
And he pushed back on suggestions that China might take objection to the meeting between Mr Obama and Asean leaders.
"I do not see this as determining the situation in the South China Sea. Asean values its relationship in America and this is an opportunity for us to meet. Naturally we will talk about security issues like the South China Sea. But at the same time, we talk to the Chinese as well and we have meetings with (them)."