A British Royal Marine and two children have died after a suicide attack on a patrol in southern Afghanistan, the Ministry of Defence said.
Royal Marines have been patrolling throughout Helmand
The deaths took place in Lashkar Gah, capital of Helmand province, after a bomber targeted a Nato vehicle.
Officials said another marine from 45 Commando was very seriously injured.
Meanwhile, a policeman has died in an attack in Khost province. The Taleban and their allies have been blamed for a rise in the number of suicide attacks.
The explosion in Lashkar Gah took place in the town centre, near the governor's compound.
The attacker, said to have been on foot, threw himself at the convoy of British troops as it was leaving the Afghan National Police Station.
Seven civilians were also reported to have been wounded.
"Our thoughts are very much with the families, friends and colleagues of those who were killed and injured today in this cowardly and indiscriminate attack," said Brigadier Jerry Thomas, commander of the Helmand Task Force.
"We should not forget that innocent Afghans going about their daily business were also injured, including two children who were killed today."
Brig Thomas said the incident had not deterred troops from their mission to support the government of Afghanistan in providing security and reconstruction.
Helmand province has seen increased violence between insurgents and Nato-led and Afghan forces this year.
The BBC's Alastair Leithead the marine was airlifted to hospital after being injured in the attack but died later from his wounds.
Lashkar Gah, in the south of Helmand, has generally been one of the safest areas in the province, our correspondent added.
The death brings to 41 the total number of UK military personnel killed while on operations in Afghanistan since 2001. The MoD said 21 died from accidents, illness or non-combat injuries.
'Do not forget'
The official spokesman for UK Prime Minister Tony Blair said: "Clearly, our thoughts are with the troops and the families of the Afghan children who were killed."
However, he insisted that, despite the attack, "we are making real progress in Afghanistan".
In eastern Khost province one policeman was killed when a suicide attacker threw himself at a convoy of police cars, officials said.
Speaking on BBC Radio 4's Today programme, Jaap de Hoop Scheffer - secretary general of Nato - stressed the need for a presence in Afghanistan.
He said: "Do not forget where the country came from.
"We are there to defend our basic values, the basic values which guide UK society, Dutch society, Belgian society. If we fail again, then Afghanistan will come to us. It will be a breeding ground for terrorists again.
"Do not forget 9/11. Do not forget other terrorist attacks. And, being in London now, I do know what it means. Britain has been a victim of this."
On Wednesday, reports said that up to 21 civilians had died during two Nato operations.