In the immediate aftermath of NATO's air war against Yugoslavia, Western officials were caught off guard when a small contingent of Russian troops dashed into Kosovo and occupied the Pristina airfield.

NATO authorities averted a military confrontation with Moscow and eventually negotiated a way of incorporating Russian forces into the NATO-led peacekeeping operation. But the June 12 incident provoked a sharp clash within NATO itself, with the British three-star officer who heads the Kosovo force resisting orders--issued by the American four-star general who commands NATO--to block the Russians.

Details of the conflict emerged yesterday at a Senate hearing on the nomination of Gen. Henry H. Shelton for a second term as chairman of the Joint Chiefs of Staff. Shelton confirmed the episode publicly for the first time.

He said the British officer, Lt. Gen. Michael Jackson, had opposed instructions from Gen. Wesley Clark, NATO's top military officer, to move military vehicles onto the Pristina airfield ahead of the Russians' arrival. Clark wanted to prevent the 200 initial Russian troops, who were rolling into Pristina by land, from flying in reinforcements.

But Jackson was worried that any precipitous NATO action could risk a major blowup with the Russians and upset the whole peacekeeping plan, which was just getting underway with the withdrawal of Serbian forces from Kosovo.

"General Jackson said: 'No, I'm not going to do that. It's not worth starting World War III.' I believe that was the quote that was used," Shelton recalled.

Jackson appealed to senior British military and political authorities in London, who persuaded Clinton administration officials in Washington to drop support for Clark's plan. Shelton, who had favored occupying the airfield, said he was personally lobbied in a 4 a.m. phone call by Gen. Sir Charles Guthrie, his British counterpart.

Asked to recount the story by Sen. John W. Warner (R-Va.), chairman of the Armed Services Committee, Shelton joined the senator in expressing concern that allowing subordinate NATO commanders to disregard the instructions of superiors could undermine alliance operations.

Warner suggested that such behavior may be all too prevalent in NATO. He noted that a NATO term even exists for it--invoking a "red card"--a phrase not written in any alliance law or regulation, but representing "some sort of a practice or understanding that has grown up through the years in NATO," Warner said.

"And now we hear of a subordinate commander failing to carry out the specific orders of the supreme allied commander, which to date and presumably for the future is an American officer," Warner said. "I find that troubling. Do you find that troubling?"

"Yes, sir, I do," Shelton replied. "As, of course, in any military operation, one of the things that we stress is discipline.

"The troubling piece," the general went on, "is that unity of command and moving in a cohesive manner--and with a chain of command that is effective--is at the heart of this issue. And certainly we can't have second-guessing at every level of command."

Warner said he intends to investigate NATO's command procedures during hearings on the future of the alliance scheduled for later this year.

Staff writer Dana Priest contributed to this report.