FALLUJAH, Iraq - U.S. warplanes and helicopters hammered gunmen today, straining a truce in besieged Fallujah. With more troops killed, April became the deadliest month for American forces since they set foot in Iraq.

To the south, a 2,500-strong U.S. force massed on the outskirts of the holy city of Najaf for a showdown with a radical cleric, raising fears of a U.S. attack that would outrage the nation's relatively pro-U.S. Shiite majority.

Russia said it would evacuate hundreds of its workers from Iraq after an outbreak kidnappings in which at least 22 foreigners still are held captive.

U.S. troops were holding back their full firepower on both fronts to allow Iraqis to try to negotiate a resolution, but President Bush said he was prepared to send more troops and had told his commanders to be ready to use "decisive force."

"Our work may become more difficult before it is finished," Bush said Tuesday night. "No one can predict all the hazards that lie ahead or the cost that they will bring. Yet, in this conflict, there is no safe alternative to resolute action."

With the announcement of the deaths of four more Marines, April became the deadliest month for the U.S. military in Iraq - deadlier even than the invasion launched in March 2003. At least 87 Americans have been reported killed in action in less than two weeks. Previously, November had seen the most deaths, 82.

Some 880 Iraqis have been killed this month. Among those are more than 600 Iraqis - mostly civilians - killed in Fallujah, according to the city hospital's director.

A U.S. Cobra attack helicopter fired rockets and heavy machine guns before dawn at gunmen gathered on the northern edge of Fallujah. Rocket-propelled grenades streamed up toward the helicopter and a second gunship providing support, but none apparently hit their target.

A-130 gunships pounded a row of buildings from which Marines say ambushes have repeatedly been launched in a residential area of the city.

A day earlier, Marines came under two heavy ambushes, the best-coordinated and largest guerrilla operations in days, said Capt. James Edge. Two Marines were killed Tuesday and two Monday, the military said.

"I think they are absolutely taking advantage" of the truce, said Lt. Col. Brennan Byrne, commander of the 1st Battalion, 5th Regiment, on the southern side of the city.

A force of 20 insurgents attacked a Marine position in a residential neighborhood, then damaged an armored vehicle that came to support it, Edge said.

A fierce battle followed to extract the vehicle as F-15s overhead fired on gunmen. Early today, A-130 gunships fired on a row of buildings in the area from which ambushes have repeatedly been launched, Edge said.

Outside the city, an MH-53 Pave Low helicopter - used to ferry special operations soldiers and large enough to carry 38 troops plus a crew of six - was hit by ground fire early Tuesday. A Marine team that came to secure it was ambushed and suffered casualties.

The Marines called a halt to offensive operations on Friday to allow negotiations between U.S.-allied Iraqis and Fallujah representatives. Gunmen in the city called a cease-fire Sunday. But Marines have been responding to guerrilla fire - and striking gunmen who appear about to attack.

Insurgents today offered the Iraqi equivalent of $7,000 for anyone who kills Mouwafak al-Rubaie, the Iraqi national security adviser, after he called for Fallujah residents to hand over militants to the United States.

"We announce a bounty of 10 million Iraqi dinar for whoever brings the head of this pig," the statement said.

Two masked militants gave the statement to an Associated Press photographer at the scene of an attack on an oil tanker near Baghdad International Airport. The tanker blazed nearby as the gunmen handed over the statement.

In the south, Iraqi politicians and ayatollahs tried to negotiate a solution to avert a U.S. attack on Najaf, home to one of Shiite Islam's holiest shrines.

A vehemently anti-U.S. cleric, Muqtada al-Sadr, was holed up in his office in Najaf, shielded not only by gunmen but by the presence of the city's main shrine only yards away. He vowed to continue what he called "a popular revolution" to end the U.S. occupation.

"I fear only God. I am ready to sacrifice my blood for this country. But I call on the Iraqi people not to let my killing put an end to their rejection of the (U.S.) occupation," al-Sadr told Lebanon's Al-Manar television station.

U.S. commanders vowed to kill or capture al-Sadr, though officials suggested they would give negotiations a chance.

"The target is not Najaf. The target is Muqtada al-Sadr and his militia," said Brig. Gen. Mark Kimmitt, deputy head of U.S. military operations in Iraq. "We will hunt him down and destroy him. We would prefer it not in Najaf or Karbala. We have very great respect for the shrines, for the Shiites."

Peacekeepers in a Polish-led force came under fire in three separate incidents overnight in Najaf and Karbala, but no injuries were reported, Lt. Col. Robert Strzelecki said Wednesday.

The U.S.-led coalition has been fighting on several fronts across Iraq this month - against al-Sadr's militia in the south, against Sunni insurgents in the central city of Fallujah, as well as increased violence in Baghdad and elsewhere.