NATO will begin its annual nuclear exercise on Monday, the military alliance's Secretary General Mark Rutte said on Thursday.

"In an uncertain world it is vital that we test our defense and that we strengthen our defense so that our adversaries know that NATO is ready and is able to respond to any threat," Rutte said in London following meetings with British Prime Minister Keir Starmer and Ukrainian President Volodymyr Zelenskyy.

The exercise will start on Monday and last for about two weeks, according to officials from the 32-member transatlantic military alliance.