The US Commerce Department added the companies to a so-called Entity List last week, restricting them from obtaining US technologies in a move blasted by Beijing on Monday as "illegal unilateral sanctions".

Washington shot down a giant Chinese balloon that passed through US airspace this month, and has accused Beijing of running a "fleet" of spy airships spanning five continents.

Here's a closer look at the six companies targeted on Friday:

- Beijing Nanjiang Aerospace Technology -

Established in 2015, Beijing Nanjiang is controlled by a subsidiary of Shanghai-listed real estate company Deluxe Family Co Ltd, which also invests in materials and robotics projects.

The state-run Science and Technology Daily in 2015 hailed the firm's development of a large silver helium airship as the country's first "new near-space platform with capabilities for both military and surveillance use".

State media said the company's steerable, reusable and continuously powered airship was equipped with broadband communications and "high-definition observation" gear.

- China Electronics Technology Group Corporation 48th Research Institute -

Part of a state-owned IT giant, the research institute specialises in building power systems and solar energy components, as well as semiconductor equipment.

The institute has worked to develop flexible solar power cells suitable for both military and civilian aircraft, the China National Space Administration said in a document in 2017.

Parent company China Electronics Technology Group Corporation also funds Hikvision, a surveillance camera maker that has been implicated in intensified monitoring of the Uyghur minority in Xinjiang.

- Eagles Men Aviation Science and Technology Group Co -

Founded by military aircraft expert Wu Zhe, the group specialises in research and development of stealth aircraft technologies.

Eagles Men is "devoted to becoming a benchmark business for China's (strategy of) military-civil fusion", according to the company's profile page on the official Chinese Society of Aeronautics and Astronautics website.

The company in 2013 filed a patent for making airship skins stronger.

Wu told state media in 2019 that his team had developed a stratospheric airship able to "fly around the globe".

- Dongguan Lingkong Remote Sensing Technology Co -

Set up in 2019, the company counts among its investors a branch of the state-run Beihang University, as well as Eagles Men Aviation.

Public records show Dongguan Lingkong has received licences from local market supervisors to conduct research on remote sensing technology, which allows aircraft to detect conditions on the ground from a high altitude.

- Guangzhou Tian-Hai-Xiang Aviation Technology Co -

The company was originally established by the Chinese military to develop "vehicle-mounted unmanned reconnaissance aircraft", according to its official website.

Specialising in surveillance drones, the company was reorganised in 2006 with its current name and under the control of military veteran Li Yuzhuang.

Tian-Hai-Xiang says it has received multiple defence science awards, with its website boasting that the company was "the first unit in the domestic drone industry to equip our military's first digitalised troops".

- Shanxi Eagles Men Aviation Science and Technology Group Co -

A wholly owned subsidiary of Eagles Men Aviation, the company was set up in 2012 with a focus on chemical products, according to Chinese business database Tianyancha.