The president of the National League of Families of American Prisoners and Missing in Action in Southeast Asia said yesterday she was "pleased to see that the government, after eight years of little effort, is finally taking some positive steps to investigate" some of the more than 300 reports of Americans being held captive in Souteast Asia.

But Ann Griffiths said that the U.S.-sponsored Laotian mercenary mission that failed to confirm the existence of Americans at one jungle prison campsite "didn't rule out the existence of American POWs in other areas."

Meanwhile, the Defense Department pledged yesterday to do "whatever is appropriate" to run down reports about Americans still imprisoned in Indochina and to bring "to an end any captivity."

Henry E. Catto Jr., assistant secretary of defense for public affairs, stated the pledge in fielding questions about the secret U.S. mission into Laos.

Catto, while declining to comment on the specific mission described in Thursday's Washington Post, said "it would be very iffy to try to speculate on what would be appropriate" actions for the government to take in regard to Americans missing in action.

The group of Asians financed and trained by the United States, who went into Laos earlier this month, failed to find any evidence that there were Americans in a jungle compound within Laos.

"Nobody in there needed a passport," said one defense official.

The reconnaissance team, which sources said was a force of 20 to 30 men, mostly Laotians, came back to Thailand with photographs of the camp, sources said.

U.S. photo interpreters this past weekend went over those pictures along with earlier photographs taken by satellite and SR71 Blackbird spy planes over the last several months. The experts concluded there were no Caucasians in the camp.

Yesterday at a Pentagon news briefing Catto said: "We have found absolutely no credible evidence of the existence of any MIAs. If new evidence should come up in the future, I can assure you that any appropriate action will be taken . . . but there is no evidence at this time that there are any Americans being held."

Griffiths and several other members of the National League of Families have been supportive of a 26-man private rescue mission that trained for several weeks in Florida this March before disbanding for lack of funds. James G. (Bo) Gritz was to lead the force from Thailand into Laos this spring before the heavy monsoon rains began. The goal was to rescue American prisoners the league believed were being held prisoners.

Gritz, 42, a retired ex-Green Beret lieutenant colonel who once led hundreds of Asian soldiers -- Lao, Cambodian, Vietnamese and Chinese -- on secret reconnaissance and ambush missions in Vietnam, scoffed yesterday in a telephone interview at the faith Pentagon analysts put in the intelligence provided by the Lao mercenaries. He said that from his jungle experience with unsupervised soldiers, the intelligence information they gleaned was usually "worthless."

"Just because they [the Lao mercenaries] say there are no Americans there, doesn't mean a damn thing," said Gritz. "They could have gone off in the bush and had an all-day sing for all we know."

Gritz said his men stand ready to go to a site where "we have convincing information from our sources to refute the [Lao mercenary] reports that Americans are still there. The POWs we believe are still there are pleading for somebody to come get them out."