Relatives of some of the six Americans held hostage in Lebanon met with President Reagan for 15 minutes late yesterday, presenting him with a yellow ribbon for the White House door, and later called the meeting "constructive."

"We do feel the president is committed" to seeing the missing Americans freed and that "he does understand the pressures" on the families and the captives, said Paul Jacobsen, son of hostage David Jacobsen, in a brief news conference outside the White House.

Jacobsen, 54, was director of the American University Hospital in Beirut when kidnaped.

"We're appreciative of the fact that they chose finally to share with us" what initiatives the administration is taking, said Peggy Say, sister of Terry Anderson, 38, chief Middle East correspondent for The Associated Press.

The relatives declined to discuss details of their meetings with the president.

In a lengthier meeting, national security affairs adviser Robert C. McFarlane reaffirmed to the families that U.S. representatives in the Middle East are "willing to talk directly with the captors" if they seek such contact, Jacobsen said, although the White House has said it will not negotiate with terrorists.

"We're waiting to see if the gesture will be picked up. We're giving a message," said Sue Franceschini, sister of the Rev. Lawrence Jenco, 50, a Roman Catholic priest kidnaped in January.

At a Rose Garden ceremony earlier in the day, Reagan responded to a question about what he intended to tell the families by saying, "All the things that we continue to do to try to bring about their relatives return."

The families have made several requests that they be granted a White House meeting with the president, but this was the first time the answer was yes.

Asked why Reagan agreed this time, White House spokesman Larry Speakes said the president "decided it would be timely for him to see them."

Franceschini said "public pressure" aided in Reagan's decision.

Yesterday's meeting followed a period of considerable violence in the Middle East and the reported killing of one of the six captives, William Buckley, 57, a U.S. Embassy political officer kidnaped March 16, 1984.

On Oct. 4, the terrorist group Islamic Jihad announced that Buckley had been killed. Eight days later, it released a photograph said to be that of his corpse. The White House has been unable to confirm that the photograph depicts Buckley's body or that he is dead. Because of increased anxiety among other hostages' family members based on these accounts, one White House official said, it was decided that Reagan should meet with them.

Buckley has no known family.

Asked if there were progress on efforts to free the captive Americans, Speakes repeated the administration's standing line:

"We are continuing to work with all parties in the region in order to bring about some resolution of this matter, and it remains a priority with the administration."

The families were in town to keep interest alive in their relatives' plight.

The other missing Americans are Peter Kilburn, 60, librarian at the American University of Beirut; and Thomas Sutherland, 54, Scottish-born dean of agriculture at the university.