Rebel Iraqi cleric Moqtada Sadr has warned coalition troops will face suicide attacks if they attack the holy city of Najaf.
The US faces a dilemma over attacking Sadr's followers in Najaf
"... We will resort to suicide operations and we will be human time-bombs," Mr Sadr told followers.
He issued the warning at prayers in Kufa, outside Najaf, where he has been since US troops surrounded the city.
The US has vowed to kill or capture Mr Sadr and destroy his Mehdi army which has been fighting coalition forces.
Najaf is one of the most sacred cities for the world's Shia Muslims, where one of the major figures of the faith, Imam Ali, is buried.
If US military planners make a deliberate decision not to attack the city, armed Sadr supporters could turn it into their stronghold for resistance.
But should the US decide to enter the city it could run the risk of inflaming the Shia world, especially if the Imam Ali shrine is damaged.
"We will shed blood to keep our holy city," Mr Sadr said.
"Lots of believers, men and women, came to me and asked permission to become martyrs and to execute martyrdom operations.
"I keep telling them to wait. But if there was an assault on our cities or on our religious authorities we will be time bombs and will not stop before destroying enemy forces."
In another sign of the growing pressure on the coalition, news agency AFP reported prominent Sunni leader Sheikh Ahmad Abdel Ghafur Samarrai warning of a possible national uprising if US forces resumed an offensive on the flashpoint city of Fallujah.
A top US commander has said Iraqi fighters in Falluja have "days not weeks" to hand over their weapons otherwise US forces will renew their offensive.
A force of about 1,500 marines continues to stop what the US says are about 200 foreign fighters from escaping.
On Friday, Colonel John Coleman, chief of staff of 1st Marine Expeditionary Force, told reporters a battle in Falluja could shape the future of Iraq.
"As Falluja goes, so goes central Iraq, as central Iraq goes, so goes the nation. Falluja is the centre of gravity."
In other developments:
A Bulgarian soldier was killed when Iraqi gunmen open fire on a Polish-led multi-national convoy through Karbala.
- An Italian soldier was shot and wounded while guarding a polling booth in the town of al-Gharraf, near Nasariyah, where municipal elections were due to be held.
- Germany's Interior Minister, Otto Schily, said two German security specialists whose convoy was ambushed in Iraq earlier this month are almost certainly dead.
- In Washington, senior Republic Senator John McCain said he believed 10,000 more American troops should be sent to Iraq.
- US Defence Secretary Donald Rumsfeld has said it is not appropriate or necessary to reintroduce military conscription.