BRUSSELS (Reuters) – Recent incidents regarding balloons shot down by the United States, which suspects they are surveillance balloons from China, form part of a pattern which highlights the need for NATO to be vigilant, said NATO Secretary General Jens Stoltenberg.

“What we saw over the United States is part of a pattern where China and also Russia are increasing surveillance activities on NATO allies,” Stoltenberg told reporters on Monday.

U.S. military fighter jets on Sunday shot down an octagonal object over Lake Huron, the Pentagon said, the latest incident since a suspected Chinese surveillance balloon put North American security forces on high alert.

It was the fourth flying object to be shot down over North America by a U.S. missile in a little more than a week. China’s foreign ministry said it had no information on the latest three flying objects shot down by the United States.

(Reporting by Bart Meijer, Benoit Van Overstraeten, Andrew Gray, Sabine Siebold and Charlotte Van Campenhout; Editing by Sudip Kar-Gupta)