Baghdad has warned suicide missions against coalition forces will become "routine military policy", following a deadly attack against US soldiers on Saturday.
The US says Iraqi troops are posing as civilians
Iraqi Vice-President Taha Yassin Ramadan said an army officer had carried out the "martyrdom operation" which killed four American soldiers - the first such attack against US forces since the war began.
The attacker - dressed in civilian clothes - drove a taxi to a checkpoint near the central city of Najaf and, as the soldiers approached it, detonated it.
The BBC's Gavin Hewitt, who is with US forces in Iraq, says it is a worrying development for the troops, who are already having to contend with sniping and other attacks.
This is just the beginning
Taha Yassin Ramadan
In other developments:
- Baghdad comes under the most concentrated bombing in more than a week.
- President George W Bush says the Iraqi Government "controls only a small portion" of the country.
Najaf has been the scene of intense fighting
- US warplanes bomb a building in Basra, where about 200 paramilitaries loyal to Saddam Hussein were believed to have gathered.
- Iraq's Information Minister Mohammed Saeed al-Sahaf says coalition troops are deliberately destroying foodstores in Basra.
- An Iraqi missile lands near a shopping centre in Kuwait City, slightly injuring two people and causing some damage.
- Iraqi troops vacate more territory in northern Iraq, which is taken by Kurdish fighters.
Iraqi television said President Saddam Hussein had awarded two posthumous medals to the suicide bomber - junior officer Ali Hammadi al-Namani.
It said the attacker wanted to "teach the invaders a lesson in the same manner as our Palestinian martyrdom fighters".
The soldiers killed were from the US Army's 1st Brigade, 3rd Infantry Division, US Captain Andrew Wallace said.
He said that when the driver stopped at the checkpoint, he indicated to the soldiers that he needed some help.
"As they approached the car... he set off the bomb," he said.
"This is just the beginning," the Iraqi vice-president warned at a news conference in Baghdad.
"It will be routine military policy. We will use any means to kill our enemy in our land and we will follow the enemy into its land," Mr Ramadan said.
Major-General Victor Renuart at US Central Command said they were concerned about any kind of "unconventional attack" on US forces.
"These kind of actions are symbolic of an organisation that is getting a little bit desperate," he said.
The US army's Major-General Stanley McChrystal said the attack would not affect the overall operation or rules of engagement though some changes might be made.
"Clearly when you see a tactic like this, it requires strict adherence or adjustments to your tactics, techniques and procedures to ensure that places like checkpoints are not vulnerable," he said.
Prior to the military campaign, there were warnings of possible suicide attacks.
In mid-March, Iraqi Foreign Minister Naji Sabri told an Arabic television station that tens of thousands of Iraqi men and women were ready to be martyrs for any war against the American enemies.
Baghdad under fire
Najaf is a small town on the Euphrates river, just some 120 kilometres (75 miles) south of Baghdad.
It has been the scene of intense fighting as US troops confront Iraqi forces defending the approaches to the Iraqi capital.
On Wednesday, US commanders said several hundred Iraqi troops had been killed when they tried to ambush the US 7th Cavalry between Najaf and Karbala.
Iraqis are appalled at civilian losses
On Saturday, the outskirts of southern Baghdad sustained one of the heaviest bombing since the wear began 10 days ago.
US bombers were believed to be targeting units of the Republican Guard defending the city.
The bombing followed an explosion at a marketplace on Friday, which killed more than 50 Iraqis.