The families of seven Americans still held hostage in Lebanon planned a gathering here next week as one of the hostages, the Rev. Benjamin Weir, entered his 444th day in captivity yesterday. That is the length of time 52 American hostages were held in Iran in 1979-81.

Noting the milestone, Weir's wife, Carol, expressed discouragement with the Reagan administration's efforts to secure the release of her husband and the others who have been kidnaped in Beirut over the last 18 months.

" Former president Jimmy Carter probably lost the election over this issue. But he at least negotiated . . . . I don't see any negotiation going on now," she said.

The families of the seven had pressed the Reagan administration to negotiate for their release along with the 39 Americans freed after the TWA hostage-taking in June.

Families of most of the seven plan to gather Tuesday on Capitol Hill to meet with members of Congress and push the administration to do more, according to Weir and others involved.

The administration has refused to talk about any negotiations for the missing Americans but has said repeatedly it is doing all it can.

Besides Weir, those held are William Buckley, 56, political officer at the U.S. Embassy in Beirut, abducted in March 1984 and the only hostage held longer than Weir; Peter Kilburn, 60, librarian at the American University in Beirut; the Rev. Martin Lawrence Jenco, 50, director of Catholic Relief Services in Lebanon; Terry A. Anderson, 37, chief Middle East correspondent for the Associated Press; David Jacobsen, 54, administrator of the American University Hospital in West Beirut, and Thomas Sutherland, 54, dean of the agriculture school of the American University in Beirut.

Meanwhile, a top State Department official told Congress yesterday that Syrian President Hafez Assad is working to free the seven Americans.

Richard W. Murphy, assistant secretary of state for Near Eastern affairs, told a House panel that "we have not forgotten those remaining prisoners in Lebanon, nor have we eased our efforts to obtain their release. I wish I could report to you that this is imminent. I cannot."

Murphy said U.S. officials believe the hostages are being held by a splinter group of Shiite Moslem fundamentalists. He said there was no way to be certain that all seven are alive, but "that is our information."

Syria's influence on the kidnapers "has its limits," Murphy said. "They're dealing with a very shadowy fragmented group . . . and they don't want to deliver seven corpses. They've found it very difficult to work out, but I think they're working on it."