WASHINGTON (Reuters) - The United States added six companies to its trade restriction list on Tuesday, including four for their links to the training of China's military forces, according to a government posting.

Two of the companies are in China, one in South Africa, two in the United Arab Emirates and one in Britain.

Global Training Solutions Limited and Smartech Future Limited, both in China, as well as Grace Air (Pty) Ltd and Livingston Aerospace Limited of the United Kingdom, were added over their ties to the Test Flying Academy of South Africa and the training of China's military forces using Western and NATO sources, the posting said.

The Test Flying Academy of South Africa was put on the Entity List, as it is known, last year for training Chinese military pilots using Western and North Atlantic Treaty Organization sources.

UAE companies Mega Fast Cargo and Mega Technique General Trading were added for evasive conduct, and Mega Fast Cargo for shipping U.S.-origin goods to Russia, the posting said.

Companies on the Entity List, which is overseen by the U.S. Commerce Department, require licenses to ship U.S. goods and technology to them, which are likely to be denied.

"Preventing American know-how from training PRC military pilots and U.S. technology from aiding Russia enhances U.S. national security," Commerce official Matthew Axelrod said in a statement. PRC refers to the People's Republic of China.

The companies could not immediately be reached for comment.

The Commerce Department also added 13 companies to its Unverified List, including eight from China, because U.S. export control officers could not complete on-site visits to determine whether they could be trusted to receive U.S. origin technology and other goods.

U.S. exporters are required to conduct additional due diligence before sending items to companies on the Unverified List, and may have to apply for more licenses.

At the same time, the department removed eight companies from the Unverified List, including six from China, one from the United Arab Emirates and one from Russia.

Both the Entity List and Unverified List are tools the U.S. is using to stop sensitive American goods and technology from falling into the wrong hands.