The State Department warned North Korea yesterday that four infiltration raids by land and sea into the south in the past five days could endanger the fragile unification talks between the two countries.

Department spokesman Hodding Carter called the flare-ups, three of which involved casualties and fire-fights with South Korea forces, a "meaningful increase" in infiltration incidents.

Such incidents had subsided since early this year when the current round of North-South talks began aimed at eventually setting up a meeting between the prime ministers to talk about unification and other issues.

To emphasize its concern, the department also made a last-minute change in a speech on U.S.-Asian policy by Assistant Secretary of State Richard Holbrooke to the Women's National Democratic Club here.

"It is difficult for us to reconcile this kind of activity on the part of the North with sincerity in the North-South talks," he said.

The new negotiations were set up largely at the initiative of the North Korean government. Holbrooke said, however, that these serious new incidents also "underlined dramatically" the possibility that the tentative new "peace feelers" may prove to be an illusion or deception, as have other such initiatives in the past.

Carter referred to ground action by a small band of heavily armed infiltrators on March 23, the sinking two days later of a North Korean vessel, described as a "spy boat" by the South, a fire-fight Wednesday night on the South Korean side of the demilitarized zone separating the two countries, and an apparently unsuccessful infiltration attempt on March 26 in which there was no contact.

Some of these reports are based on South Korean information, but Carter said he had no reason to doubt them.

Both Carter and Holbrooke, however, said they supported continuation of the talks.