A militant Palestinian group has declared that its first wave of volunteer suicide bombers has arrived in Iraq.
Arab volunteers: Already in Baghdad
Al-Quds Brigades, the military wing of Islamic Jihad said in a statement that it "brings to our people and nation the good news of the arrival of its first martyrdom (attackers) to the heart of Baghdad."
The group's mission is "to fulfil the holy duty of defending Arab and Muslim land," by attacking coalition troops, the statement said.
Iraq has declared that more than 4,000 foreign Arab volunteers have come forward to fight and were ready to die if necessary.
A spokesman for Islamic Jihad in Lebanon said that the potential attackers had come from several Arab states, not from the Palestinian territories.
Army spokesman General Hazem al-Rawi told a news conference in Baghdad suicide missions against US and British soldiers would be stepped up as the war continues.
His comments come a day after a group of US soldiers in the central city of Najaf were killed in a suicide car bombing.
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.
We are a believing people, a fighting people, Jihad is a must, a duty ordained by God,
Iraqi army spokesman
General Hazim al-Rawi
The Iraqi spokesman also challenged the US account of casualties in the incident.
"Eleven, not four Americans were killed," he said, and dozens injured in what he described as the first on the glorious path of Jihad (holy war) against the invaders,"
Earlier, Vice-President Taha Yassin Ramadan said such attacks would become "routine military policy".
"We will use any means to kill our enemy in our land and we will follow the enemy into its land," he said.
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.
Najaf has been the scene of intense fighting
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 US enemies.