Two U.S. strategic bombers conducted drills over South Korea, the U.S. Air Force said, raising tensions with North Korea just days before President Donald Trump visits the region seeking to shut down Pyongyang’s nuclear program.

While North Korea has not launched any missiles since Sept. 15, the longest such lull this year, a flurry of activity has been detected at its missile research facilities in Pyongyang, pointing to another possible launch, South Korea’s spy agency told lawmakers on Thursday.

Seoul held a National Security Council meeting on Friday to discuss possible unilateral sanctions against Pyongyang, and may announce the measures ahead of Trump’s arrival in South Korea, a presidential official said.

“The United States has wanted the South Korean government to take further steps to sanction the North. Unilateral sanctions by South Korea wouldn’t have much practical impact but have a symbolic importance,” the official said.

Ahead of Trump’s visit to Asia, the drills involving the U.S. bombers marked the latest show of force by the United States and its allies.

The planes flew over the Pilsung Range training area in central South Korea, Air Force spokeswoman Captain Victoria Hight said.

Japanese and South Korean fighter jets carried out sequenced missions with the U.S. bombers but no ordnance was dropped, the Air Force said.

The drill was staged at a time when three U.S. aircraft carrier groups are in the Asia-Pacific, the first time such a potent projection of force has been together in the region in a decade.