Syrian Ambassador Rajik Jouejati, in a joint news conference yesterday at his embassy here with the Rev. Jesse L. Jackson, told reporters that Syria has been doing "all it could" to find out who is holding five Americans captive in Lebanon and to press for their release.

"The results so far are negative, that is, there are no results. But we are not despairing," Jouejati said. "We cannot be accused of any negligence. We have nothing to do with this problem but, for humanitarian purposes, we shall continue to work for their release."

Jouejati and Jackson met for 30 minutes and discussed the captives in the broader context of the need for, in Jackson's words, "a new level of respectful dialogue" for Mideast peace.

Jackson said he has not determined when he might visit Syria to attempt a repeat of his feat of December 1983, when he negotiated release of a captured U.S. Navy flier. Jackson has vowed to use his good relations with the Syrians to work toward release of the captives.

"The commitment by this Syrian government to do whatever it can certainly is positive," he said, calling this week's emergence of a videotape of one of the captives "a source of inspiration."

Jackson described Reagan administration efforts on the captives as "inadequate" and called for an internatonal peace conference involving all nations with an interest in the Middle East conflict. The Soviet Union has proposed such a conference, but the United States has opposed it as premature and not likely to be fruitful. Israel also is opposed.

The administration has adopted a policy of not talking about specific efforts to free the captives, but has said it is pursuing contacts with the Syrians and a variety of others.

The captives, kidnaped from the streets of Moslem-dominated West Beirut in the last year, include U.S. diplomat William Buckley, who appeared on the videotape; Jeremy Levin, Cable News Network bureau chief in Beirut; the Rev. Benjamin Weir, a Presbyterian minister; Peter Kilburn, a librarian at the American University of Beirut; and the Rev. Lawrence Jenco, a Roman Catholic priest who headed the Catholic Relief Services office in Beirut.

Anonymous telephone callers claiming to belong to the terrorist entity known as Islamic Jihad have said the group is holding the five and have threatened to try them as spies.

Jackson has kept in touch with some of the captives' relatives and plans to meet with them in Chicago today.