FALLUJAH, Iraq, Nov 8 (AFP) - As a battle for Fallujah opens, a psychological war has already been raging between the US military and fighters in the rebel enclave of western Iraq.

Insurgents claimed they had captured US and Iraqi soldiers, with Islamist leaders announcing over loudspeakers in a city mosque that 36 Americans and 107 Iraqi national guardsmen had been seized.

"Let us announce with joy the capture," the rebel claim said, adding that the prisoners would be paraded publicly and a video of their humiliation released.

Insurgents in southwestern Fallujah had also shot down a US helicopter, a symbol of America's military superiority over the often lightly-armed insurgents, said rebel leader Khaled Hammud Jumali.

For its part the US military, without confirming the claims, has accused the rebels of terrorizing the local population.

"Terrorist groups in Fallujah and Ramadi have increased their use of intimidation tactics and violence against innocent Iraqi citizens," the military said in a statement.

Putting further pressure on Iraqi soldiers -- often recent recruits to the country's fledgling military -- a leading Sunni religious group Monday warned them against taking part in the fight for Fallujah.

"The participation of Iraqi forces with the invaders against a Muslim city is a great sin that will provoke the anger of God against them," the Committee of Muslim Scholars declared Monday.

"We call on all Iraqi forces -- the national guard and others -- not to participate under the banner of the (Americans)."

Ali Hussein Jumali, a leader with the rebel group Mujahadeen of Fallujah, said Iraqi soldiers must desert or face "extermination".

Iraqi Prime Minister Iyad Allawi acknowledge Monday that there had been "one or two" instances of soldiers fleeing, but said "that is normal in military operations all over the world" and dismissed reports of mass desertions.

Allawi on Monday afternoon gave the formal go-ahead for the 20,000 US-led multinational troops camped around Fallujah to launch the assault.

At the same time, he announced a raft of emergency security measures, including a curfew in the city and the temporary closure of the airport and the borders with Jordan and Syria.

US-led forces seized a hospital and two bridges on the western edge of the city overnight but clashes with the insurgents holed up in Fallujah were fierce, with a barrage of rocket, mortar and gunfire raining down as they tried to raise the new Iraqi flag above the hospital.

About 230 Iraqis were found by marines still living in a building complex in the northwest edge of the city, some milling about in their underwear.

The military has accused insurgents of preventing Fallujah residents from leaving the city before it is engulfed by the fighting.

"Residents of Fallujah informed Multi-National Forces recently that terrorists in the city are preventing families from leaving Fallujah," the military said.

"According to residents, terrorist elements plan to use citizens as human shields then claim they were attacked by Multi-National Forces."

The military also claimed that insurgents would use mosques and schools to transport arms and stage attacks.

During the press conference where he announced the assault on Fallujah, Allawi showed maps he said were found on foreign fighters that proved the insurgents would violate holy buildings.

He also showed pictures of arms and explosives he said were found in a mosque and youth center in the flashpoint town of Ramadi.

"They planned to bomb the Al-Haq mosque and blame the US-led forces. They (insurgents) say they are against the multinational forces but they kill the Iraqi people.

"These forces are bent on destroying Iraq. They think Iraq is weak but I warn them from this podium their time has come."