The United States and South Korea are holding their biggest joint military exercises in years.

The Ulchi Freedom Shield training operation opened Monday in South Korea. The exercises are happening during increasing tensions with North Korea, which has sharply increased weapons testing this year. Last month, its leader, Kim Jong Un, again threatened to use nuclear weapons in reaction to conflict with South Korea and the U.S.

The joint exercises will continue through September 1. The training involves airplanes, warships, tanks and thousands of troops.

The U.S. and South Korea say their military preparations are for defensive purposes. But North Korea describes the two countries as training for an invasion.

American and South Korean forces reportedly will simulate efforts such as combined attacks, resupply of arms, and removal of nuclear weapons.

The allies will also train for drone attacks and other new developments in warfare.

Tensions between the U.S. and North Korea have grown since the failure of a meeting between Kim and former President Donald Trump in 2019. The U.S. rejected North Korea's offer to close an old nuclear center in exchange for an end to U.S. sanctions.

Kim has promised to strengthen his nuclear weapons abilities since that time.

South Korean and U.S. military officials have not said how many troops are taking part in Ulchi Freedom Shield.

The exercises follow North Korea's dismissal last week of South Korean President Yoon Suk Yeol's proposal of economic support in exchange for ending its nuclear program. North Korea accused South Korea of using old proposals that have been rejected already.

Kim Yo Jong is the powerful sister of North Korean leader Kim Jong Un. She called Yoon's proposal foolish. She said her country has no desire to surrender its nuclear weapons. And, last week, she warned of "deadly" retaliation against South Korea over a recent North Korean COVID-19 outbreak.

There are concerns that the threat could lead to nuclear or missile tests or even small fights at the border.

Last week, North Korea launched what experts suspect was two cruise missiles. So far in 2022, North Korean weapons testing has included more than 30 ballistic missile launches, including the country's first launches of intercontinental ballistic missiles (ICBMs) in almost five years.

Experts say North Korea is increasing its testing so that it can negotiate economic and security demands from a position of strength. There are also some signs that it is preparing to test nuclear weapons.

I'm Dan Novak.

Dan Novak adapted this story for VOA Learning English based on reporting by The Associated Press.

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simulate — v. to look, feel, or behave like

drone — n. a type of small aircraft that flies without a pilot

sanction — n. an action that is taken or an order that is given to force a country to obey international laws by limiting or stopping trade with that country, by not allowing economic aid for that country, etc.

ballistic missile — adj. a weapon that is shot through the sky over a great distance and then falls to the ground and explodes