The United States has agreed to sell four armed MQ-9 Reaper drones to Taiwan, the State Department announced Tuesday, helping to boost the island's defenses as China steps up its threats.

The $600 million sale aids Taiwan's "continuing efforts to modernize its armed forces and to maintain a credible defensive capability," the State Department said.

It will also assist in maintaining political stability and the military balance in the region, the department said in a statement.

The sale covers four drones, ground stations, and associated surveillance and communications equipment, but not the bombs or missiles usually associated with it.

The drones deal comes on the heel of several other major arms packages to Taiwan announced in recent weeks worth $4.2 billion, including potent Harpoon anti-ship missiles, air-launched SLAM-ER cruise missiles, air reconnaissance technology and mobile light rocket launchers.

"This is the tenth arms sales to Taiwan under President Trump and the third time in two weeks that the US government has supplied our country with major defensive weapons that will enable Taiwan to be more capable and confident in defending peace in the Taiwan Strait," Taipei's foreign ministry said in a statement.

The sales have angered Beijing, which regards Taiwan as its own territory and has vowed to one day seize it.

Relations between Taiwan and China have plummeted to their worst levels in decades.

In 2016, Taiwan elected President Tsai Ing-wen, who views the island as an already sovereign state and not part of "one China".

China cut off official communication and piled on economic, military and diplomatic pressure in response, although Tsai was reelected in a landslide in January.

President Xi Jinping has become the most bellicose Chinese leader since Mao, describing the seizure of Taiwan as "inevitable".

Chinese jets have in recent months begun crossing into Taiwan's defence zone at an unprecedented rate.

While Taiwan has for decades fallen back on an implicit US security guarantee, Washington has urged it to strengthen its own capabilities.

The US military has used the unmanned Reapers, made by General Atomics, for long, high-altitude reconnaissance missions and pinpoint attacks and assassinations of jihadists in the Middle East and Afghanistan.

The sale is the first after the US government decided in July to diverge partly from the 1987 Missile Technology Control Regime, in which 35 countries agreed to restrict the sales of unmanned weapons delivery systems.

The decision was to permit the export of medium-speed drones like the Reaper that had been blocked by the agreement.