US troops in Iraq have launched new air raids against insurgents in Falluja as President George W Bush vowed to do everything to "secure" the city.
Hundreds of civilians have been displaced
For a second night an AC-130 gunship bombarded rebel positions in the city.
Mr Bush said military commanders would take whatever action was necessary to eliminate "pockets of resistance".
But UN Secretary General Kofi Annan urged restraint. Mr Annan said if civilians were harmed, the anti-coalition resistance would grow.
US marines besieging the city say they have called in air support only after coming under enemy fire.
However, US forces answer attacks with punishing ferocity, reports Jennifer Glasse from a US marine base outside Falluja.
US commanders say they want to avoid an all-out assault on Falluja - but one top army general has suggested the troops may in any case not have enough heavy armour.
In a change of policy, heavily-armoured combat vehicles, including Abrams tanks and Bradley personnel carriers are being sent to the front line.
The deployment reflects the inadequacy of the standard military personnel transporter in Iraq, the Humvee, to deal with roadside bombs and rocket-propelled grenades.
The BBC Pentagon correspondent says the change of strategy seems to bolster the argument of those who say the US military failed in its planning.
Plumes of smoke rose again after nightfall on Wednesday in the restive city which - at the heart of the so-called Sunni triangle - has spearheaded opposition to the US-led occupation.
Earlier, US forces dropped 10 large laser-guided bombs against buildings guerrillas were firing from, Lt Col Brennan Byrne said.
Without specifying what measures US forces would take, Mr Bush said commanders on the ground "have got the authority necessary" to secure Falluja.
"Most of Falluja is returning to normal. There are pockets of resistance and our military along with Iraqis will make sure it's secure," said Mr Bush.
From New York, a blunt warning was issued by Mr Annan.
"Violent military action by an occupying power against the inhabitants of an occupied country will only make matters worse," Mr Annan told a news conference at UN headquarters.
"It's definitely time now for those who prefer restraint and dialogue to make their voices heard," Mr Annan said.
There are signs the White House shares this concern, says the BBC's Rob Watson in Washington.
But reports suggest that may be putting it at odds with some in the US military who feel their hands are being tied by the politicians in Washington, our correspondent says.
There are fears US soldiers might not have enough heavy armour
The US military operation in Falluja, 50km (30 miles) west of Baghdad , began on 5 April following the gruesome killings of four American security contractors in the predominantly Sunni Muslim city.
US military spokesman Brig Gen Mark Kimmitt said on Wednesday that "Falluja is more than a military problem" and talks with local leaders were continuing.
However, the spokesman said the coalition remained concerned that "these leaders who we remain engaged with are not capable of delivering".