Iraq has closed a border crossing point with Syria as US and Iraqi forces wage gun battles with insurgents in the northern town of Talafar.
The assault went ahead after days of fighting failed to dislodge rebels
The move came as US and Iraqi troops swept through Talafar, smashing walls with armoured vehicles.
Iraq's defence ministry said more than 140 insurgents had been killed in the town in the last two days.
The Americans believe the town is being used as a staging post by foreign fighters crossing into Iraq from Syria.
The Rabiah border crossing - about 100km (60 miles) east of Rabiah - is now closed to all vehicles except those with special permission from the interior ministry.
Iraqi and American authorities have complained that the Syrians have done too little to stop the flow of foreign fighters crossing into Iraq to join the anti-US insurgency.
The BBC's Jonathan Charles in Baghdad says the operation in Talafar is the biggest against rebels since the assault on Falluja last year.
The assault on Talafar has been expected for some time and the authorities earlier urged Talafar's 200,000 residents to leave.
It has been the scene of heavy fighting and US air strikes in the past week.
Iraqi Prime Minister Ibrahim Jaafari said action had to be taken in Talafar because insurgents were trying to isolate it from the rest of Iraq and the political process.
"At 0200 [local time] today, acting on my orders, Iraqi forces commenced an operation to remove all remaining terrorist elements from the city of Talafar," Mr Jaafari said in a statement.
"These forces are operating with support from the Multi-National Force," he said.
The US military drove the insurgents out of Talafar a year ago, only for them to return once the troops had withdrawn.
Iraqi Defence Minister Saadoun Dulaimi said 144 insurgents had been killed during the operation in Talafar in the last two days and 192 captured. Many of them were Arabs from other states.
He said attention would be turned to other parts of the country which needed help.
"We tell our people in Ramadi, Samarra, Rawa and Qaim that we are coming," Mr Dulaimi told a news conference.
"There will be no refuge for the terrorists, criminals and bloodsuckers."
The Iraqi Red Crescent has set up camps outside Talafar to receive civilians trying to escape the fighting.
"There is bombing missiles and tanks bombing everywhere in sight," says Dr Salam Ismael Obaidi, of the Doctors for Iraq organisation.
Mr Obaidi told the BBC that families fleeing the fighting were being separated at checkpoints outside the city, with women and children sent to refugee camps and some young men arrested at random and taken away.
But Adnan Ali, a spokesman for the Iraqi prime minister, insisted the government had taken necessary measures to avoid loss of innocent life.
He added that those who had not committed any crime would be granted amnesty.
"We are calling on them, we are distributing leaflets through the aircraft, requesting the civilians to leave," Mr Ali told the BBC.