Two U.S. aircraft carriers kicked off joint exercises in the Philippine Sea on Sunday, a day after Southeast Asian leaders delivered some of their strongest remarks opposing Beijing’s claim to virtually the entire South China Sea on historical grounds.

The USS Nimitz and USS Ronald Reagan Carrier Strike Groups began the drills to bolster the United States’ “responsive, flexible, and enduring commitments" to mutual defense agreements with allies and partners in the Indo-Pacific, the Navy said in a statement.

The dual carrier exercises also came exactly a week after the Nimitz and another carrier, the USS Theodore Roosevelt conducted their own joint operations in the area. It is rare to see three U.S. aircraft carriers operating at the same time in the Western Pacific and even more unusual to have separate dual carrier exercises within such a quick time frame.