Four North Korean patrol boats crossed a disputed sea border after the communist state demanded South Korea stop what it called provocation in waters off the peninsula's west coast and threatened to strike the South's forces there.

"The South Korean authorities must know that if they continue reckless provocations despite our repeated warnings, they will meet with our strong self-defensive strikes. There is a limit to patience," the (North) Korean Central News Agency reported.

The situation was extremely grave, the North's agency said, quoting the spokesman for the mission of its Korean People's Army in the truce village of Panmunjom.

"It is an unshakable will of the Korean revolutionary armed forces never to pardon those who violate . . . the sky, the land and the sea of the fatherland, in defense of their sovereignty," it added.

The report said South Korea must be held responsible for all consequences of the naval "provocations," which it saids rendered the situation in Korea extremely dangerous.

South Korea massed more destroyers, frigates and even landing ships near the area after putting its 650,000 military on heightened alert Saturday. It canceled weekend leaves for soldiers. Both states, technically still at war after their 1950-53 conflict ended in a truce but not a peace agreement, have been in confrontation over fishing waters within the zone for five days.

On Friday, naval vessels from the North and the South trained their guns on each other and nearly opened fire, a lawmaker said. Ships from the South rammed those from the North, forcing them away.

The incident Friday began when six North Korean warships took up positions inside the disputed area, escorting about a dozen fishing boats. South Korean ships rammed four of the North's boats, causing "substantial damage" to two of them, which had to be towed away, said Col. Hwang Dong Kyu, spokesman for the South Korean Joint Chiefs of Staff. One of the South Korean ships had a hole in its bow, he added.

The ships trained their guns on each other and it nearly became an armed clash, South Korean lawmaker Han Young Soo said.

The disputed waters lie south of a U.N.-imposed sea border, midway between the North Korean mainland and five South Korean islands, 60 miles northwest of Seoul.