TEL AVIV, Israel — Israel’s relationship with Turkey, its closest ally in the region, has been put under severe strain by the Israeli army’s discovery that one route Iran used to resupply Hezbollah in Lebanon ran through Turkey into Syria.

The intelligence, described by one Israeli official here as “irrefutable,” found that in the days prior to the August 14 cease-fire, a shipment of spare parts and components for mobile missile launchers was sent by truck through Turkey to Syria and then into northern Lebanon before being funneled down to front-line Hezbollah terrorists in southern Lebanon.

An American diplomat and another Israeli official who requested anonymity confirmed this account last night.

The information that Turkey, a NATO ally and a candidate for membership in the European Union, allowed the transshipment of Iranian arms to Hezbollah has caused so much concern in Washington that a formal representation to the Turkish government was made in both Ankara and Washington to express America’s displeasure.

The American diplomat, however, noted that since America made a formal complaint to Turkey, the Bush administration is now satisfied that the Turks will monitor their border with Iran more aggressively.

Evidence suggests that Turkey has done just that. On Sunday, the Turkish military boasted to a newspaper, Hurriyet, that based on tips from American and Israeli intelligence, it had grounded Iranian planes en route to Syria.

On Monday, Iran’s Foreign Ministry complained publicly about three instances when Turkey grounded Iranian flights.

The prospect that Turkey may have tried to curry favor with Iran during Israel’s war with Hezbollah is all the more significant as Turkey is currently weighing sending its soldiers to become part of an international force in Lebanon that would, at least in theory, enforce an arms embargo on Hezbollah.

Yesterday, the Turkish foreign minister, Abdullah Gul, said in Damascus, “We believe that after every tragedy, there is a chance to achieve peace and that every party should have learned a lesson from the last events in the region,” according to a dispatch from United Press International.

Until the international force is in place, Israel is taking no chances. Prime Minister Olmert told U.N. envoys in Jerusalem yesterday that Israel would maintain its air and sea blockade until there was an international troop presence in southern Lebanon to enforce the terms of U.N. resolution 1701, which established the cease-fire between Israel and Lebanon.

The American diplomat said, “Nobody was aware of” shipments through Turkey “before the war started, not us or the Israelis. It may have been a reaction by Hezbollah to find a new route. The Israelis raised it directly with the Turks. By the time we were raising it with the Turks, they said they did not know about it and asked us to help shut this down.”

He added, “They told us that this was rough terrain. If they had known about it, they would not have let it happen. I think it would be a mistake to say they were allowing the shipments deliberately.”

While the diplomat said America is satisfied that the Turks will be more vigilant in the future, some Israelis remain concerned that Turkish officials were aware of the Iranian shipments and did nothing to stop them. No officials were willing to discuss the shipments on the record yesterday because of the sensitive relationship Turkey has with Israel, but two officials did stress that Israel shared intelligence with Turkey on many of the shipments coming through.

“This is a complicated situation,” one Israeli official said.

Turkey is Israel’s primary trading partner and a key source of fresh water for the Jewish state. Both militaries have a close relationship and the two countries also share intelligence.

At the same time, however, the Justice and Development Party government that controls the government in Ankara is made up of devoted Islamists. Prime Minister Erdogan was the first head of state to invite Hamas representatives to his country after it won Palestinian Arab parliamentary elections in January.

One of those representatives was Khaled Meshaal, the chief of Hamas’s terrorist wing, who is widely believed to have authorized and planned the June 24 kidnapping of an Israel Defense Force soldier, Corporal Gilad Shalit.

Last month, Mr. Erdogan publicly defended Yassin al-Kadi, who has been designated by the State Department, the United Nations, and the European Union as a financier of Al Qaeda. The Turkish press reported that Mr. Kadi’s associate, Faruk Sara, has contributed to Mr. Erdogan’s campaigns.

A former Pentagon Iran and Iraq analyst and a scholar at the American Enterprise Institute, Michael Rubin, said, “During the Cold War, America had a special relationship with Turkey. After the Iraq war, Turkey was just an ordinary ally. Increasingly under Erdogan, rather than being an ally in the war against terrorism, Turkey is a liability.”