BRITISH journalist John McCarthy, held hostage in Lebanon since April 1986, was released by the Islamic Jihad organization yesterday, carrying an important message to deliver personally to United Nations Secretary-General Javier Perez de Cuellar.Two UN sources close to the secretary-general expect the message to set terms for a comprehensive hostage release under UN auspices. The UN could provide channels of communication between all the parties and a face-saving formula for a release, they say. The UN chief took no offense at an accusation earlier this week by anonymous Beirut bombers that he had "godfathered" a sudden flurry of moves that led to at least the one hostage release. Mr. Perez de Cuellar said that he was only too happy to play such a role: "I have been working for six or seven years to obtain the release of all the hostages." Though the UN will apparently be little more than a silent observer in a Middle East peace conference to be convened in October, the secretary-general may be playing a crucial role in paving the way for the talks by helping to arrange the release of hostages held by all sides in the area. He is to travel to Geneva this weekend for talks with UN agency heads on humanitarian operations in Iraq. Iranian diplomats have been pressing the UN chief to travel to Tehran to discuss the unfinished UN business on the Iran-Iraq war. They say they were told that the visit would be combined with a Perez de Cuellar trip to Europe - but an announcement has been inexplicably delayed. There is speculation that the UN chief is holding up his visit to Iran until there is movement on a hostage release. One of the secretary-general's top aides, Giandomenico Picco, arrived in Damascus yesterday for talks with Syrian officials and with the visiting Iranian Interior Minister Abdollah Nouri. He was reportedly bound for Lebanon. Mr. Perez de Cuellar said only: "My people are in the place I need them." The No. 2 diplomat in the United States mission to the UN, Alexander Watson, was also in Damascus. He told journalists that his trip was on bilateral US-Syrian matters unrelated to a hostage release. But there is speculation that Mr. Watson, who recently spoke in the UN Security Council about US concern for Shiites under siege by the Iraqi Army, may be exploring preliminary contacts with the Iranian official. In addition to the 12 Westerners still being held hostage, there are also: * Four Iranian diplomats who disappeared at a checkpoint manned by Christian militiamen in Beirut nearly a decade ago. * Seven Israeli soldiers captured in June 1982 during military operations in Lebanon. * A leading Shiite cleric, Sheikh Abdel Karim Obeid, kidnapped two years ago by Israeli forces trying to curb the activities of the radical Hizbullah group in south Lebanon. * Thousands of Lebanese and Palestinians seized since the breakout of the Lebanese civil war in 1975, and held by rival Lebanese and Palestinian militias, as well as by the Israel-backed South Lebanon Army and by Israel itself. A UN office in Beirut was bombed Wednesday - without damage or casualties - by a group that threatened greater displeasure if Western hostages were released while their far more numerous Lebanese and Palestinian counterparts were not. Israeli officials said yesterday they were open to discussions on a hostage exchange. Israel released about 70 Lebanese and Palestinian prisoners to mark an important Islamic holy day last month. Despite intensive speculation at the time, Sheik Obeid was not among them. The Islamic Jihad has insisted that the cleric be freed - and so, apparently, has the US State Department, which is also said to have expressed strong criticism in private to Israel about the abduction. A Syrian Foreign Ministry official said Friday that the UN was in a position to expedite contacts with the Israelis. A communique signed by Islamic Jihad and distributed to news agencies in Beirut on Tuesday, accompanied by a previously unreleased photo of Associated Press Middle East Bureau Chief Terry Anderson, promised that an envoy would bring a message to the UN within 48 hours. The Tehran Times predicted Tuesday that two Western hostages would be released by the weekend. Two UN employees were reportedly hanged by their captors. A videotape showed British journalist Alec Collett, on a short-term writing assignment for the UN Relief and Works Agency for Palestine Refugees, hanged shortly after the 1986 US bombing of Libya. The British Foreign Office has never confirmed Mr. Collett's death. American Lt. Col. William Higgins, a marine officer on secondment to the UN Peacekeeping operation in Lebanon (UNIFIL), was also shown hanged on videotape a few years later.