Gunmen shot down a U.S. attack helicopter during fighting in western Baghdad (search) on Sunday, killing its two crew members. Insurgents and Marines called a cease-fire in the besieged city of Fallujah (search), but the fragile peace was shaken by shootings that wounded two Americans.

Smoke rose on Baghdad's western edge where the AH-64 Apache (search) helicopter was downed by ground fire in the morning. More helicopters circled overhead, while U.S. troops closed off the main highway — a key supply route into the capital.

Brig. Gen. Mark Kimmitt told a news briefing Sunday that the two-member crew was killed and a quick-reaction team was collecting the bodies.

Heavy firing was heard, and tanks and Humvees moved into the area near the suburb of Abu Ghraib, where masked gunmen have wreaked havoc for the past three days, attacking fuel convoys and blowing up tanker trucks. Insurgents kidnapped an American civilian and killed a U.S. soldier in the area Friday.

President Bush, attending an Easter service at a chapel at Fort Hood, Texas, braced the country Sunday for the possibility of more American casualties in Iraq while saying the U.S.-led mission is just.

"It was a tough week last week and my prayers and thoughts are with those who pay the ultimate price for our security," the president said. "Our troops are taking care of business. Their job is to make Iraq more secure so that a peaceful Iraq can emerge."

The captors of Thomas Hamill, a Mississippi native who works for a U.S. contractor in Iraq, threatened to kill and burn him unless U.S. troops end their assault on Fallujah, west of Baghdad, by 6 a.m. Sunday. The deadline passed with no word on Hamill's fate.

Video footage aired on Arabic television Sunday showed the bodies of two dead Westerners — possibly a pair of Americans seen by APTN cameramen on Friday being dragged out of a car on the Abu Ghraib highway, in a different incident from Hamill's kidnapping.

The video showed the bodies surrounded by gunmen, who say the two were American intelligence officers. One of the bodies lay sprawled on the pavement, his face bloodied and his right leg drenched in blood. The other body has his shirt lifted to reveal a bullet hole in his back. Both wore dark T-shirts and khaki pants often worn by private contractors.

Other insurgents who kidnapped two Japanese men and a woman said Saturday they would free their captives within 24 hours because of an appeal from Sunni clerics. But the two aid workers and a photojournalist had still not been released by Sunday evening, according to a Japanese Embassy official in Baghdad, Hiroyuki Oura.

Also Sunday, Germany's Foreign Ministry said two security agents for the German Embassy in Iraq are probably dead four days after their convoy was ambushed near Fallujah. And Britain's Foreign Office said Gary Teeley, a British man who had reportedly been kidnapped in the southern city of Nasiriyah, is safe and in the hands of coalition forces.

Fallujah — 35 miles west of Baghdad — saw occasional sniper fire Sunday, but was the quietest it has been all week. Sunni insurgents and Marines agreed to a cease-fire that started early Sunday and will last until the evening amid talks between Iraqi officials on how to end the violence.

More than 600 Iraqis have been killed in the fighting in Fallujah since fighting began early Monday, the head of the city's hospital said Sunday. Rafie al-Issawi said actual number may be higher because there were reports of people being buried at home.

At least five Marines have died in the fighting.

Members of the Iraqi Governing Council were holding a second day of negotiations with city representatives Sunday in an attempt to win the handover of Iraqis who killed and mutilated four American civilians on March 31 and of other militants.

Hundreds of U.S. reinforcements moved in place on the city edge, joining 1,200 Marines and nearly 900 Iraqi security forces already involved in the fighting.

The most serious break in Sunday's peace came when a sniper opened fire on U.S. patrol, wounding two Marines, commanders said. In the ensuing gunbattle, at least one insurgent was killed.

"They are not playing by the rules, sir," Marine Capt. Jason Smith radioed to his commander after taking fire in another incident in which the troops did not fire back.

Sunday was the first that gunmen have said they were joining the halt in offensive operations that Marines have largely stuck to unilaterally since noon Friday.

"At the moment we're just trying to get the cease-fire in place," L. Paul Bremer, the top U.S. civilian administrator in Iraq, said Sunday on ABC's "This Week." "What were trying to do is simply get the forces to stop firing."

About a third of the city's population of 200,000 fled Friday and Saturday, though Marines turned back any military-age men trying to leave, said Lt. Col. Brennan Byrne, commander of the 1st Battalion, 5th Marine Regiment.

During the lull, Marines distributed food to residents.

"Families are holed up in houses. They have been told to stay inside. But they are running out of water and food," said Marine Capt. Jason Smith, 30, from Baton Rogue, La.

In fighting across the country since April 4 — including in Fallujah and in the uprising by the militia of radical Muslim cleric Muqtada al-Sadr in the south — 47 American soldiers and nearly 900 Iraqis have been killed. At least 649 U.S. soldiers have died in Iraq since the war began in March 2003.

In southern Iraq, some 1.5 million Shiite pilgrims marked one of their holiest religious days, al-Arbaeen. In the city of Karbala, hundreds of Shiite militiamen — but no police — patrolled the street preparing for a possible U.S. assault against rebellious followers of al-Sadr.

U.S. commanders have suggested they will move against al-Sadr, whose militia has control of Karbala and two other cities, after the al-Arbaeen ceremonies, which mark the end of a 40-day mourning period for a 7th-century martyred Shiite saint.

In other violence:

— Gunmen ambushed Iraqi police before dawn Sunday in the northern city of Kirkuk, sparking a battle joined by U.S. troops. Four attackers were killed, said Iraqi Col. Sarhad Qadir.

— Insurgents attacked two Iraqi police patrols in Mosul on Saturday in fights that killed two Iraqi police, a gunman and two passers-by, according to the hospital.

— Armed men clashed with U.S. soldiers in the Sunni neighborhood of al-Azamiyah in Baghdad on Saturday. Four Iraqis were killed.