Three boats have exploded in apparently co-ordinated suicide attacks near two Iraqi oil terminals in the Gulf, off the southern city of Basra.
Attacks on oil supplies have plagued Iraq's rebuilding process
Two American sailors were killed and five were injured when their vessel tried to intercept one of the boats.
These are the first maritime attacks on Iraqi oil installations since the March 2003 US-led invasion of Iraq.
They came at the end of a grim day of attacks and explosions across Iraq, in which scores of people died.
One official termed the boats which apparently attempted to strike at Iraq's oil terminals - crucial for the country's oil exports - as "suicide speedboats".
The small boats, or dhows, were apparently attempting to reach the Bakr and Khawr al-Amaya oil terminals.
An eight-man coalition interception team was approaching one of the boats when it exploded, flipping the coalition boat and killing two crew members, said the US navy's Bahrain-based Fifth Fleet.
Five crew were also injured, said a spokeswoman.
The two other boats apparently exploded some minutes later.
However, no damage was done to either of the terminals, US and UK defence officials said - the apparent target of the attackers.
The Bakr terminal - also known as the Basra terminal - lies about 15 kilometres (9 miles) south of Iraq's main port, Umm Qasr.
As a target, the terminal had everything, says the BBC's Lucy Williamson. Not only politically important, the oil exported through the southern port of Basra is also economically crucial in the country which badly needs investment.
Scores died in attacks that pockmarked Iraq on Saturday
It is also symbolically linked in the minds of many Iraqis to the reasons behind the invasion, she says.
The attack appeared to resemble the October 2000 bombing of the warship USS Cole off Yemen, when 17 US sailors died.
In that incident, blamed on al-Qaeda, two suicide bombers rammed a small boat packed with explosives into the side of the ship as it was refuelling.
Also on Saturday in Iraq:
- At least 13 Iraqis were killed and 30 wounded in explosions in the Sadr City suburb of Baghdad.
- A roadside bomb killed 14 Iraqis and wounded at least 11 travelling on a bus en route to Baghdad near Iskandiriya.
- At least five US soldiers died and seven were wounded - three critically - when two rockets were fired into the US base at Taji, north of Baghdad. Another two US soldiers were reported to have died in an attack on their convoy near Kut, south of Baghdad, but the attack has not been acknowledged by the coalition.
- In Tikrit, the former home town of ousted Iraqi leader Saddam Hussein, at least three Iraqi policemen were killed and 16 - including 12 police officers - were wounded when a car bomb exploded near the main US base.
- And in Karbala, south of Baghdad, five insurgents were killed by Polish soldiers, a Polish contingent spokesman said.