JERUSALEM — Israel and Syria announced on Wednesday that they were engaged in negotiations for a comprehensive peace treaty through Turkish mediators, a sign that Israel is hoping to halt the growing influence of Iran, Syria's most important ally, which sponsors the anti-Israel groups Hezbollah and Hamas.

Senior Israeli officials from Prime Minister Ehud Olmert's office and their Syrian counterparts were in Istanbul, Turkey, on Wednesday, where both groups had been staying separately, at undisclosed locations, since Monday. The mediators shuttled between the two. Syria and Israel have not negotiated this seriously in eight years.

Syria's motives are clear: It wants to regain the Golan Heights captured by Israel in the 1967 war and to re-establish a relationship with the United States, something it figures it can do through talks with Jerusalem. For Israel — which has watched the Palestinian group Hamas take over Gaza and gain ground in the West Bank, and the Lebanese group Hezbollah display raw power in Beirut, Lebanon, in recent weeks — an effort to pull Syria away from Iran could produce enormous benefits. An announcement on Wednesday of a peace deal that gives Hezbollah the upper hand in Lebanon's government probably added to Israel's sense of urgency on the issue.

The American government opposed Israeli-Syrian negotiations because they feared that such a negotiation would reward Syria at a time when the United States is seeking to isolate it for its backing of Hezbollah and its meddling in Lebanon, Bush administration and Israeli officials said. The United States yielded when it became clear that Israel was determined to go ahead, they said.

The talks come less than a week after President Bush, speaking to the Israeli parliament, created a stir by criticizing those who would negotiate with "terrorists and radicals."

The Turkish prime minister, Recep Tayyip Erdogan, has been working on convening negotiations for some time, an official in his office said, including holding phone conversations with leaders on both sides, and assigning a special envoy to handle the diplomatic back-and-forth. The fact that messages were being exchanged has been public for a couple of months, because of official Syrian statements.

The senior Israeli official said that shortly after Olmert became prime minister more than a year ago, he went to Turkey and held a long one-on-one meeting with Erdogan in which it was decided that Turkey would mediate between Israel and Syria.