The United States has deployed a P-8 Poseidon spy plane to Singapore for the first time.
It is the latest in a series of US military actions seen as a response to China's increasingly assertive claims over territory in the South China Sea.
The US says it will also base a military reconnaissance plane at Singapore's Paya Lebar air base.
US P-8s already operate from Japan and the Philippines, and surveillance flights have taken off from Malaysia.
The P-8 was deployed on Monday, and will remain in Singapore until 14 December.
In addition to the P-8 deployment, the US says it will operate a military plane, either a P-8 Poseidon or a P-3 Orion, from Singapore for the foreseeable future, rotating planes on a quarterly basis.
The US-Singapore agreement, announced after a meeting in Washington on Monday between US Defense Secretary Ash Carter and Singapore Defence Minister Ng Eng Hen, also covers co-operation on counter-terrorism, fighting piracy, and disaster relief.
Speaking at a daily news briefing, Chinese foreign ministry spokeswoman Hua Chunying said the US was "pushing regional militarisation", which did "not accord with the joint long-term interests" of the region.
Despite rising tension with China over its South China Sea territorial claims, a US navy spokesman based in Singapore insisted that the deployments were "not solely focussed on the South China Sea".
"It's not about the South China Sea, it's about partnership with Singapore and other partners in the region," said Lt Commander Arlo Abrahamson, speaking to the BBC.
A US navy statement also said that the deployment was "in support of shared maritime security and HA-DR (High Availability and Disaster Recovery) initiatives in the Indo-Asia-Pacific region."
China also has a separate dispute with Japan over islands in the East China Sea.