Two supersonic U.S. bombers flew over South Korea on Wednesday — with one landing on the Korean Peninsula for the first time in 20 years — amid soaring tensions after the North’s fifth nuclear test on Sept. 9.

The U.S. B-1B landed at Osan air base 40 km (25 miles) south of the capital, the air force said in a statement. It said the flight, which took off from Andersen Air Force Base in Guam, was the closest a B-1B strategic bomber had ever flown to the border between South and North Korea.

South Korea’s Yonhap news agency reported that the aircraft flew over a U.S. live-fire training site in the Pocheon area bordering the North.

U.S. Forces Korea did not say how long the jet would stay at the base. It said the flight was conducted as a demonstration of Washington’s “ironclad” commitment to maintaining security on the peninsula.

“The bond between the United States and the Republic of Korea is ironclad and the strength of that commitment will not be shaken by North Korea’s aggressive behavior,” said Lt. Gen. Thomas W. Bergeson, 7th Air Force Commander. “What we are showing today is just one tool we have to choose from a wide array of options.”

South Korean Air Force Operational Commander Lt. Gen. Lee Wang-kuen said the North’s nuclear test, as well as its continued ballistic missile launches had prompted a “grave security crisis” on the peninsula.

“Should the enemy provoke us once again, the Combined Air Forces will respond and eliminate their will and capability to fight,” Lee Wang-kuen was quoted as saying.

The United States, which has about 28,500 troops in South Korea, flew two B-1B bombers on Sept. 13 escorted by U.S. and South Korean fighter jets in a show of solidarity with Seoul.

Originally developed for nuclear capacities, the bomber was converted to its exclusively conventional combat role in the mid-1990s. It is capable of carrying the largest payload of both guided and unguided weapons in the air force inventory.

The B-1B, capable of reaching the Korean Peninsula from Guam in just two hours, is one of the U.S. military’s three major multirole, long-range bombers along with the B-52 and B-2.

There was also a possibility that the U.S. will send B-52s and B-2s to Korea as well, Yonhap said Wednesday. The B-52 can carry nuclear missiles and “bunker buster” bombs that are capable of destroying underground facilities.

According to Euan Graham, director of the International Security Program at the Lowy Institute in Sydney, the most noteworthy aspect of the flights was not the nearness to the North Korean border, but rather than a nonnuclear jet is currently being tasked with the role of nuclear reassurance.

“The U.S. is trying to calm South Korean nerves, to deter the North Koreans — and not raise tensions with China,” Graham said. “I suspect that’s why the B-1B … has been selected for these highly visible flights” despite Seoul having wanted Washington to bring in dual-capable B-52 and B-2 bombers after the fifth nuclear test.

Graham said this reflects Seoul’s desire for the U.S. to communicate that extended deterrence is functioning “when North Korea is off the nuclear leash and pro-nuclear weapons advocates have suddenly become more vocal” in the South.

“The U.S. does not want to go down a road that leads back to the reintroduction of nuclear weapons onto the peninsula, or which also raises tensions with Beijing,” Graham said

Pyongyang has ignored global condemnation of its fifth nuclear test, while also continuing to test-fire missiles. On Tuesday, it announced the successful ground test of a new type of “high-power engine,” ostensibly for launching satellites, but which the U.S. has called a cover for testing missile technology. In February, the North launched a satellite that was widely seen as a test of long-range ballistic missile technology.

The United States, Japan and South Korea have roundly condemned the nuclear test and called for tough new measures to further isolate the communist state.

Washington, meanwhile, has pressed Beijing, Pyongyang’s most important diplomatic backer and trading partner, to do more to rein in the country. The two countries have reportedly begun discussions on a possible U.N. resolution in response to the latest nuclear test, but it remains to be seen whether this would mean closing loopholes in earlier sanctions.

In another demonstration of force, Washington is planning to dispatch a nuclear-powered aircraft carrier to South Korea next month as part of its efforts to deter further military provocations by the North, reports have said.

The Yokosuka, Kanagawa Prefecture-based USS Ronald Reagan was reportedly scheduled to participate in a joint naval exercise with the South Korean military from Oct. 10 to 15. The exercise would focus on training the allies’ naval forces in joint precision attacks on the North’s military facilities and the regime’s leadership that would be launched in the event of a war with the isolated state.