South Korea and the U.S. launched their 11-day Ulchi Freedom Shield military exercise Monday, involving 21,000 troops in command simulations and field training.

The allies say the drills are defensive, but North Korea condemned them as invasion rehearsals and warned of retaliation.

The exercise comes as South Korean President Lee Jae Myung prepares for an Aug. 25 summit with U.S. President Donald Trump, amid concerns Trump may demand higher payments for U.S. troops or scale back the alliance.

Lee has called for restoring a suspended 2018 inter-Korean military pact to ease tensions, though Seoul says drills will continue, the Associated Press has reported.

The U.S. stresses the alliance must adapt to North Korea’s advancing weapons program, its closer ties with Russia, and growing Chinese influence.