"We call on the Iraqi forces, the National Guard and others
who are mostly Muslims ... to beware of making the grave mistake of invading Iraqi cities under the banner of forces who respect no religion or human rights," the Association of Muslim Scholars (AMS) said in a statement on Monday.
"Beware of being deceived that you are fighting terrorists from outside the country, because by God you are fighting the townspeople and targeting its men, women and children and
history will record every drop of blood you spill in oppressing the people of your nation," the AMS said.
Interim Prime Minister Iyad Allawi on Monday authorised US and Iraqi forces to begin an offensive against Falluja and Ramadi to root out foreign fighters and Saddam Hussein loyalists he says are entrenched there.
The US and the interim Iraqi government say the offensive is intended to help stabilise the country before January elections.
Poll boycott threatened
However, residents say they know nothing of the foreign fighters and daily US bombardments just bolster the local anti-US movement.
US-led forces say they will quell
Falluja before January's elections
The AMS has threatened to call for a boycott of the poll if assaults on cities in Iraq's Sunni heartland escalate.
The last time US forces stormed Falluja in April, some units of Iraq's fledgling security forces refused to fight.
Moreover, the AMS, which has helped negotiate the release of foreign captives in Iraq, warned security personnel not to repeat the "mistake" they made in joining US forces to quell an uprising in Najaf in August.