The U.S. Air Force has flown 14 F-22 stealth fighters to Yokota Air Base in western Tokyo, the Japanese Defense Ministry said Friday.

The arrival of the latest model of stealth fighters follows North Korea’s nuclear test on Jan. 6 and the victory of Tsai Ing-wen, an independence-leaning opposition leader, in Taiwan’s presidential election last Saturday. China regards the self-ruled democratic island as a renegade province.

Eight F-22s landed at the base on Wednesday, four on Thursday and two on Friday, according to the ministry.

The Japanese government was not given prior notice of the deployment of the fighter jets, Defense Minister Gen Nakatani said.

When the ministry inquired about the arrival of the fighters, the United States replied the Air Force would temporarily deploy a total of 26 F-22 and F-16 fighters at the Yokota base from Wednesday to Friday.

The deployment was part of the “usual practice of security cooperation between Japan and the United States,” Nakatani said.

“As North Korea’s nuclear test clearly shows, the security environment around our country has become increasingly severe,” he said.

“The deterrence provided by the U.S. military based on the Japan-U.S. alliance is indispensable for our country’s security.”