China is keeping a close watch on U.S. Navy ships in the South China Sea, even sending vessels to “check out” the ships and lingering for days, according to a recent report.

Two Chinese frigates watched the USS Ronald Reagan, the U.S.’s Japan-based aircraft carrier, on Saturday as it conducted routine drills according to a Bloomberg report.

In addition, the Reagan’s officers described “frequent close quarter surveillance” from Chinese People’s Liberation Army Navy ships in international waters.

Sometimes, those Chinese ships “steam in to check out the carrier.” Other times, the ships linger for days, they said. The Reagan‘s crew have had to alert the Chinese vessels on occasion when they have to sharply alter course, they said.

Rear Adm. Marc Dalton, commander of the Reagan carrier strike group—the complement of ships and aircraft that protect the carrier—said, “We’ve had no issues. They’ve been very professional.”

“We see them on a regular basis,” he said.

Dalton said he would soon lead the carrier to a port call in Hong Kong, the first such visit in three years after a previous request in 2016 was denied.

China claims much of the South China Sea as its own territory, even though parts of it are also claimed by six other countries.

The U.S. maintains the waters are open to international passage and does not take sides in the disputes, but urges nations to resolve territorial disputes peacefully through international arbitration mechanisms.

The Reagan is the only U.S. carrier permanently stationed overseas, and would likely be part of any response to any U.S. military contingency.

North Korean missile and nuclear tests have kept the region on edge in recent months, and the U.S. has kept a military option on the table to prevent North Korea from getting a nuclear bomb that can hit the U.S.

The Reagan strike group is planning to exercise with South Korean defense forces in October, according to the report.

Dalton said the carrier task force he leads is ready to execute its missions, despite a series of accidents by U.S. ships in the Asia Pacific.

“As a forward deployed force … we are already where we need to be to execute our missions all the time,” he said.

He called North Korea’s tests a “growing and concerning danger.”

“The U.S. has been very clear about leveraging all options in order to get North Korea to change its path,” he said.