BAGHDAD, Iraq - An Iranian envoy headed to Najaf today on a mission to work out a solution to the U.S. standoff with a radical Shiite cleric, an intervention by a nation Washington has tried to keep out of Iraqi affairs and a sign of the eagerness to avert a U.S. attack on the holy city.

Meanwhile, Iraqi militants executed an Italian hostage, the first captive known to have been killed among at least 22 foreigners kidnapped during Iraq's spasm of violence this month.

The wave of kidnappings has sent a chill through the foreign community in Iraq, including aid workers, journalists and private contractors.

Some 150 Russians have fled the country so far in an evacuation called for by Moscow because of the abductions, said Dimitri Elen, a Russian Embassy diplomat in Baghdad. The Russian government is sending flights to evacuate around 553 Russians and 263 citizens of former Soviet republics.

The identity of the kidnappers - apparently a variety of small groups - has been unclear. A senior U.S. official said the military and coalition did not know who they were but suspected former intelligence officials from Saddam Hussein's regime or foreign militants were behind the abductions.

In the besieged city of Fallujah, U.S. warplanes struck guerrillas early today, the latest in nightly fighting that has strained a four-day truce called to allow Iraqi negotiators to try to end the violence.

Marines and insurgents have been digging into their positions in houses inside the city, preparing for the possible complete collapse of the cease-fire. Insurgents were launching increasingly sophisticated attacks on Marine positions at night, Marine commanders said.

Two soldiers were killed Wednesday in attacks in Mosul and Samarra, in the north and center of the country, the military said. At least 89 U.S. soldiers have been killed this month - making it the deadliest month for Americans ever in Iraq. More than 900 Iraqis have also been killed, the most since the fall of Saddam.

In an effort to keep the political process moving despite the violence, a top U.N. envoy proposed that an Iraq caretaker government take power when U.S. officials hand over sovereignty on June 31.

Lakhdar Ibrahimi's plan would dump the current Governing Council and set up an executive made of highly respected Iraqis, including a prime minister, president and two vice presidents.

The plan, for the first time, would also give the United Nations a role in picking the new government, in contrast to the entirely U.S.-picked Governing Council, which many Iraqis dismiss as lackeys of Washington.

The executive would be chosen by the United Nations, the Governing Council, the coalition and a select group of Iraqi judges, according to the U.N. spokesman's office in New York. Party or ethnic affiliation would not be a factor in the choice, a distinct difference from the current council, carefully proportioned on ethnic lines.

Washington tried in the past to keep the political process almost solely between it and the Iraqis, but sharp differences over how to move forward forced it to give the United Nations a prominent role.

Now this month's intense violence was giving an entry for Iran into Iraq's affairs.

An Iranian delegation, headed by senior Iranian Foreign Ministry official Hussein Sedeqi, met Wednesday evening with Massoud Barzani, current president of the Iraqi Governing Council.

The talks were "positive" and the Iranians expressed their willingness to mediate the U.S. dispute with radical Shiite cleric Muqtada al-Sadr, said council member Mahmoud Othman. The Iranians headed to Najaf on Thursday, an al-Sadr aide said.

Some 2,500 U.S. troops are massed outside the southern town of Najaf, vowing to capture al-Sadr, located at his office next to the city's Imam Ali Shrine - the holiest Shiite site in the world.

A U.S. assault into the city could enflame Iraq's Shiite majority and push them closer to al-Sadr, whose militia launched a bloody uprising last week against coalition forces across the south. It would also fan anti-American sentiment in Shiite communities around the world, including mostly-Shiite Iran.

Iraq's top Shiite cleric, Grand Ayatollah Ali al-Husseini al-Sistani, persuaded radical cleric Muqtada al-Sadr to drop defiant demands he had put forward to Iraqi politicians currently mediating the standoff. Among other things, al-Sadr demanded U.S. troops withdraw from all Iraqi cities, a condition the U.S. military was unlikely to accept.

Al-Sadr militiamen in Najaf appeared to be preparing for a fight, moving into buildings and onto rooftops on Najaf's outskirts, said Col. Dana J.H. Pittard, head of the 2,500 U.S. troops amassed outside the city, ready to move in against al-Sadr.

"Najaf is a holy place," said Kaysal Hazali, spokesman for al-Sadr. "If they attack it, God knows the results: It is not going to be good for the occupation."

In Fallujah, the top Marine commander warned that the halt in offensive operations that the Marines have maintained since Friday may not last much longer in the face of persistent guerrilla attacks, despite a truce insurgents called on Sunday.

A senior U.S. official in Baghdad said up to 2,000 insurgents are thought to be holed up in the city, west of the capital.

"I don't forecast that this stalemate will go on for long," said Maj. Gen. James N. Mattis, commander of the U.S. military's 1st Marine Division. "It's hard to have a cease-fire when they maneuver against us, they fire at us."

Tuesday night, insurgents launched near simultaneous attacks on several positions of a company of U.S. Marines controlling a few blocks in the city's northeast. In a five-hour battle later that night, one of two armored vehicles sent to resupply a front-line U.S. position got lost during an ambush and ended up inside the southern part of city.

The vehicle, with 20 U.S. Marines inside, came under an even larger ambush. At least 100 gunmen opened fire with rocket-propelled grenades, hitting it at least 10 times, knocking out its communications and its engine and paralyzing it.

"They've been preparing for this the whole time. ... We definitely stumbled into the wasp nest," said Capt. Jason Smith.

At least 20 insurgents were killed in the battle, Marines said.

In other violence:

Gunmen attacked the house of Iraqi Electricity Minister Ayham al-Samarie, sparking a gunbattle with his guards. There were no casualties.

A car bomb exploded near a U.S. patrol outside Baqouba today, and witnesses reported U.S. casualties. There was no immediate comment from the U.S. military.