The US has flown B-1B Lancer bombers escorted by F-15 fighters off North Korea’s coast venturing the “farthest north of the Demilitarized Zone,” separating the two Koreas, in the 21st century, the Pentagon’s spokesperson said.

The planes took off from Okinawa, Japan and flew over the waters east of the Korean Peninsula.

"This is the farthest north of the Demilitarized Zone (DMZ) any U.S. fighter or bomber aircraft have flown off North Korea's coast in the 21st century, underscoring the seriousness with which we take (North Korea's) reckless behavior," said Pentagon spokeswoman Dana White.

#USAF bombers, fighters fly in international airspace east of #NorthKorea, farthest north of the DMZ in 21st century https://t.co/R5DLUQigSFpic.twitter.com/fsFx4Q1oKE — U.S. Pacific Command (@PacificCommand) September 23, 2017

The DMZ is a strip of land running across the Korean Peninsula near the 38th Parallel, separating North Korea from South Korea. It was created in 1953, following the armistice which ended the Korean War.

#PACOM stands prepared to use our full range of military capabilities to defend the U.S. homeland and our allies if called upon to do so. — U.S. Pacific Command (@PacificCommand) September 23, 2017

The B-1B Lancer strategic bombers entered service in the mid-1980s. The plane was designed specifically as a bomber for nuclear capabilities, thus having a limited capability to carry conventional bombs. Following the demise of the Soviet Union, the role of a bomber for purely nuclear war became questionable, and the Lancer fleet was grounded. The planes eventually underwent a series of modifications, which bolstered their conventional bombing capacity, but deprived them of their nuclear load.

The patrol followed a 3.4 earthquake registered in North Korea earlier on Saturday, which prompted fears of a new nuclear test. The seismic event, however, turned out to be a natural occurrence and “unlikely man-made,” according to geology and nuclear weaponry experts.

The show of force reinforced the recent threats voiced by US President Donald Trump, who vowed on Friday that Kim Jong-un “will be tested like never before,” branding the North Korean leader a “madman.”