Even though some are more than 50 years old, the North Korean government pays attention when Washington flies its strategic warplanes nearby, a top U.S. general said Thursday.

On Sunday, Jan. 10, just days after North Korea claimed it had carried out a hydrogen bomb test, the Pentagon flew B-52 over the Korean Peninsula in support of its ally to the south.

“Without revealing anything classified, the intelligence reports that I get on the North Korean reaction to the B-52 flying a low approach at Seoul say no, they do not shrug,” U.S. Air Force Gen. Paul Selva, vice chairman of the Joint Chiefs of Staff, said Thursday at a Brookings Institution event. “They actually pay attention.”

Video shows the Cold War-era bomber flanked by a South Korean F-15 and an American F-16.

The Pentagon began deploying bombers to Guam, where the U.S. maintains a sprawling air base, more than a decade ago. The deployments have become continuous over the years, said Dave Deptula, a retired Air Force lieutenant general who initiated the rotations in 2004 when he was the director of operations at Pacific Air Forces.

“It has been a critical element of conventional deterrence of ‘adventurism’ exhibited by the North Korean regime over the years, and continues to be successful in that role today,” Deptula said.

The B-52 and B-2 stealth bombers that deploy there can carry tens of thousands of pounds of conventional and nuclear weapons. Despite its age, the B-52 Stratofortress has been upgraded with a suite modern electronic equipment so it can drop guided bombs.

“The North Koreans are very aware of the presence of the aircraft and the capability that they represent,” Selva said.

In March 2013, two B-2s flew more than 6,500 miles from their home base in Missouri to the Korean Peninsula. The planes dropped inert bombs on a South Korean training range as part of Key Resolve/Foal Eagle military drills between Washington and Seoul.

In recent years, the Pentagon has launched a project to replace the B-2 and B-52 with a new stealth bomber, saying it is essential for both deterrence and the ability to strike guarded targets anywhere on the globe.

“They’re going to have to be capable of going into some of the most complex surface-to-air missile defense systems that humans have built,” Selva said said of the new planes. “That’s the requirement.”

Russia has built and sold sophisticated surface-to-air missiles, including some that it has deployed to Syria.

“Any country that wants to compete with the United States has watched the last two decades pass,” Selva said. “They have gone to school on the way that we use airpower … to empower our joint force and they have built incredibly elegant and integrated surface-to-air missile defense systems to try and keep us out.”