During a visit to Washington, Chinese President Xi Jinping told Barack Obama in 2015 that China would not militarise the man-made islands.
Yet in the intervening 20 months, Beijing has stepped up construction and now has runways that can accommodate Chinese fighter jets.
Over the past three months, China has reportedly built four new missile shelters on Fiery Cross, increasing the number of installations on the reef to 12.
China has also expanded radar facilities on Fiery Cross and two other disputed reefs — Subi and Mischief — in the Spratly Island chain.
The country has also started building underground structures which reportedly will be used to store munitions.
There was a two-day summit between President Xi and US President Donald Trump at the Mar-a-Lago estate in April.
Greg Poling, director of CSIS’s Asia Maritime Transparency Initiative, said: "We haven’t seen any slowdown in construction, including since the Mar-a-Lago summit.
"The islands are built and they are clearly militarised, which means they already got over the hard part. Now every time they put in a new radar or new missile shelter, it is harder for the world to get angry. They are building a gun, they are just not putting the bullets in yet.
Xi Jinping told Barack Obama in 2015 that China would not militarise the man-made islands
When asked about freedom of navigation and new construction in the South China Sea, a People’s Liberation Army spokesperson said there was "no problem with China’s freedom to carry out construction on its own territory”.
China’s legal claim to the seas around the maritime features is legally controversial because many were dredged out of coral and sand and therefore not entitled to island status.
Euan Graham, an Asia expert at the Lowy Institute in Sydney, said it was "not quite game over in the South China Sea”.
However, he added that China had fundamentally changed the status quo over the islands and that it would prove hard to change barring war or natural disasters.
Mr Graham said: "They already exert a strategic effect by projecting China’s presence much further out.
"They will not prevent the US Navy from operating in their vicinity, but they will complicate the threat environment for US ships and aircraft — by extending the [Chinese navy’s] surveillance and targeting net, as well as the envelope of power projection.”
Ely Ratner, an Asia expert who served in the Obama administration, admitted the US had failed to come up with an effective strategy to convince China to halt militarisation of the islands.
Mr Ratner said: "Until China believes that there will be significant costs... I don’t think they have any reason to slow down. They have been pushing on an open door and have been surprised at how little resistance they have faced.”