The United States has caught North Korea building another tunnel under the demilitarized zone separating North and South Korea, government sources said yesterday.

The military view of this newest attempt to sneak into the south is that it represents fresh evidence of North Korea's hostile intentions.

The tunnel, about 6 feet by 6 feet, could be used for surprise attack, military specialists said, or for secret forays into South Korea by saboteurs or spies, assuming the exit on the south side of the DMZ could be concealed.

The former possibility, a secret corridor for a hard-hitting force of North Koreans spearheading an invasion, is what worries U.S. military leaders the most. North Koreans could race through the tunnel at night, dressed in South Korean uniforms, they said, emerge behind the main line of defenses and attack lightly defended Seoul, the South Korean capital.

Intelligence sources said this newest tunnel is the fourth one North Korea has attempted to dig under the DMZ since 1974.They indicated that the tipoff came through seismic and other listening devices and from information obtained by the South Korean Central Intelligence Agency.

Starting in North Korea at a point northeast of Seoul, just outside the supposedly sacrosanct, 2 1/2-mile-wide DMZ, the new tunnel goes through solid granite in places. The granite muffled the sound of underground explosions and jackhammers, making it difficult to defect where North Koreans were digging.

Sources said they presume President Carter had the preliminary evidence about the new tunnel when he announced on July 20 that the plan- ned withdrawal of American troops from South Korea would be suspended until at least 1981.

The expected response by the United States and South Korean leadership is to protest this latest tunnel as an aggressive act and then move to seal it up. But protesting tunnels has been a lot easier than finding them.

Tunnel number 3 went 246 feet below the surface and stretched almost a mile from its opening behind a hill just beyond the North Korean side of the DMZ before it was discovered and plugged last year. The last 1,400 feet of the tunnel was on South Korea's side of the DMZ.

A North Korean defector reportedly informed South Koreans about the third tunnel being dug near Panmunjom, the truce talk center. The first tunnel was discovered near there in 1974.But it took 18 months of boring holes in the rock over the suspected location before the tunnel was found. And even then a lucky break gave the tunnel's location away.

North Korean engineers deep in the tunnel set off an explosion right underneath one of the bore holes of the U.N. Command's search party in June 1978, sending up a geyser of muddy water and pieces of the plastic liners in which the dynamite was set.

U.S. and South Korean engineers then proceeded to dig a tunnel of their own to intercept the North Korean one. The intercept passage went 925 feet before breaking through into the North Korean tunnel.

The U.N. Command at Panmunjom sealed off that third tunnel, put it under guard and formally protested on Oct. 27, 1978, that digging the tunnel was an "aggressive act" by North Korea.

The first North Korean tunnel was dynamited shut in 1974, and the second one was closed in 1975.

Sources here said the new tunnel is also being sealed, possibly with a thick cement plug.

This latest tunnel discovery hardens suspicions of U.S. military leaders that North Korea intends to keep digging, diplomatic protests or not.

The North Koreans, sources said, have been blatant lately about their tunnel construction, wheeling highly visible well-drilling rigs around their border area along DMZ.The well digging rigs apparently start the tunnels down into the earth on the North Korean side.

Few Korea specialists believe this fourth tunnel is the last one, predicting more discoveries in this bizarre cat-and-mouse game being played along the 155-mile-long buffer zone between the two Koreas.