SINCE the release of British hostage John McCarthy Thursday, Israel has come under increasing diplomatic pressure to make a goodwill gesture and release at least some of the 300 Shiite prisoners it controls in south Lebanon.Britain, for instance, called in the Israeli ambassador in London over the weekend to convey the suggestion. Most of the Shiite prisoners are believed to be held in south Lebanon's Khiam prison, which is run by the Israeli-backed South Lebanon Army (SLA), a 3,000-man militia patrolling the security zone. Over the weekend, Israeli officials reported that SLA commander Antoine Lahd had released 40 inmates earlier this year at Israel's request as a "unilateral show of good will." The release, announced by Uri Lubrani, Israel's policy planner for Lebanon, hadn't been public before. But he adds that no more releases would follow until Israel learns the whereabouts and condition of seven Israelis missing in Lebanon, some since 1982. "We have been consistently ... sticking to our position that we will participate in any massive release operation if we were to be satisfied that our soldiers were to be freed," Mr. Lubrani says. The Israelis believed held in Lebanon are an airman and two soldiers captured in 1986, and a four-man tank crew that disappeared in 1982. It is not known who holds them and whether they are still alive. Perhaps Israel's best point of leverage in future hostage exchanges is its own prisoner, the pro-Iranian Shiite cleric Sheikh Abdel Karim Obeid. Twenty-five commandos abducted the cleric from Jibsheet in south Lebanon in June 1989. He is believed to be held in Israel. Together with at least 300 Lebanese Shiite prisoners held in Israel and in its self-declared "security zone" in south Lebanon, the Obeid abduction may connect the release of missing Israeli soldiers to that of the Western hostages in Lebanon. A spiritual leader of Hizbullah - one of the groups believed to be holding the Westerners - Sheikh Obeid's stature has risen dramatically since his abduction, observers say. This enhanced reputation makes him a powerful bargaining chip, raising the chances of a comprehensive hostage and prisoner exchange, they note. "Obeid is a very good pressure point on Hizbullah," says David Rudge, the Lebanese affairs reporter for the Jerusalem Post newspaper. "A package deal from Israel's point of view stands a better chance of success for getting back the Israelis if it is linked to the foreign hostages," he adds. Yossi Olmert, director of Israel's government press office and a specialist in Lebanese and Syrian affairs, insists that Obeid was seized for his ties to anti-Israel terrorist groups and not to link the fate of Israeli soldiers with that of the Western hostages. "The key [to the release of Western hostages] is in the hands of those who hold the hostages, not Israel," Mr. Olmert says. He adds that the Israeli demand to include its prisoners in any exchange for Western hostages is a matter of fairness to the Israeli soldiers, not premeditated strategy. "Shall we allow all the Lebanese to leave Israel only for the sake of the Westerners and not the Israelis? I don't think anyone is saying that," Olmert says. Israel several times has swapped hundreds of Arab prisoners for small groups of its own soldiers captured in Arab countries. The latest deal in 1985 involved the exchange of 1,150 prisoners for three Israeli soldiers. There is also speculation here that the groups holding the Westerners in Lebanon will demand the release of Palestinian prisoners in Israeli and SLA jails in addition to the freedom of the Lebanese Shiites. That demand could be contained in a letter now in the possession of United Nations Secretary-General Javier Perez de Cuellar, written by McCarthy's former kidnappers and handed over yesterday. "At the moment, Israel is not talking about any Palestinians, only Lebanese," Rudge says.