The United States approved the sale of three Global Hawk surveillance drones to Japan on Friday, as Tokyo revamps its military in response to Chinese maritime ambitions.

Unless Congress intervenes within a month to halt the $1.2 billion deal, US defense giant Northrop Grumman will supply the pilotless aircraft and their sensor array.

The US Defense Security Cooperation Agency said the sale of the RQ-4 drone, already in wide use with US forces, would not tip the military balance in the region.

But it would allow a major US ally to better patrol its skies.

"The proposed sale of the RQ-4 will significantly enhance Japan's intelligence, surveillance and reconnaissance capabilities and help ensure that Japan is able to continue to monitor and deter regional threats," it said.

Under Prime Minister Shinzo Abe, Japan is building up its Self Defense Forces to counterbalance a rising China and better play its role as a United States ally.

The country has been officially pacifist since its World War II military defeat, and it was only in September that Tokyo passed a law allowing its troops to fight abroad.

Many in Japan are uneasy at the change, but this week Abe's government reinforced its tougher stance by publicly backing a US decision to sail its warships though disputed waters in the South China Sea.