The soldier died after being hit by a sniper, reported Aljazeera. US forces are reported to have encircled Sa'da village near al-Qa'im city, west of Baghdad, and arrested seven Iraqis following the incident.
In another attack in the troubled region a US vehicle was reported ablaze with all its crew killed following an explosion at Al-Anbar University.
The three British soldiers were killed and another wounded in the southern city of Basra in an attack that also left at least four Iraqi civilians wounded.
The four British soldiers were travelling in a white civilian four-wheel drive vehicle, alongside a military Land Rover, when they came under fire.
“The driver was hit and lost control of the vehicle, hitting an Iraqi woman and two children. He then slammed into a building,” a witness said.
The attack in Basra is significant in that the mainly Shia city has so far been free of the kind of resistance attacks that have become a feature of Sunni regions.
Britain’s armed forces minister struck a defiant note in the first official response from London.
"Today's attack will not deflect us from our determination to work with the vast majority of Iraqi people who want to build a secure and democratic future for the country," said Adam Ingram.
The deaths brought to 10 the number of British soldiers killed in Iraq since Washington declared major combat over on 1 May.
In the north, US forces came under fire south of Baaquba, an Aljazeera correspondent reported. US tanks and helicopters encircled the area at al-Faris airport after the mortar attack but there were immediate reports of casualties.
In the same region, US forces raided the house of Sheikh al-Shawi, chief of the Ubayd tribe in al-Hamra, 10km north of Tikrit. The troops made 15 arrests, Aljazeera reported.
UN staff returned to work at their
bomb-damaged Baghdad HQ
A member of the chieftain’s family said US forces had also confiscated several hundred dollars and some gold jewellery.
The arrests are the latest in a US crackdown in the north of the country.
The US Army's 4th Infantry Division, operating north of Baghdad in Saddam Hussein's homeland around Tikrit, have detained 35 people, including four suspects with links to the toppled government, over the last 24 hours, said Lieutenant Colonel Bill MacDonald.
UK evacuates, UN returns
Meanwhile, a large crowd gathered in al-Majar al-Kabir district in Amara province, southern Iraq, to protest against the arrest of 11 citizens by British forces, an Aljazeera correspondent reported.
Religious leaders from Hawza, the grassroots Shia Muslim organisation, have been negotiating with British forces in Najaf to get the prisoners released, and a member of Iraq's governing council is heading to the area to intervene, our correspondent added.
In Baghdad, the British Foreign Office said diplomats and staff at its embassy had been evacuated after information was received about a possible attack.
But UN employees returned to work at their headquarters in the capital, as US Army engineers continued to search the rubble for survivors of Tuesday's truck bombing.