Major Spike Kelly on the ''enormous sadness'' over the deaths
Four British Royal Marines have been killed in two separate explosions in southern Afghanistan, the MoD has said.
A Royal Marine from 45 Commando was injured in an explosion in the Sangin area of Helmand province and died on the way to hospital at Camp Bastion.
Two marines from 45 Commando and one from Commando Logistics Regiment died in a second blast south of Sangin.
Prime Minister Gordon Brown, on a trip to Afghanistan, said the men died "in the front line against terrorism".
Next of kin have been informed of the deaths, which take the total of British troops killed in Afghanistan since 2001 to 132.
The first of the latest fatalities happened while a marine was on patrol in a Jackal armoured vehicle.
BBC defence correspondent Paul Adams said the Jackal was one of a number of new armoured vehicles intended to offer British forces better protection than the controversial Snatch Landrover.
The other three died in an explosion during a routine operation against enemy forces in the area.
The blast happened as an Afghan boy pushing a wheelbarrow approached a company of British marines. It is not clear whether the boy was a suicide bomber or the device was remotely detonated.
Of those killed, one died instantly, a second died of wounds before they could be moved for hospital treatment and the third died after being taken to Camp Bastion.
Three of the four dead were from 45 Commando, based in Arbroath and currently on a six-month deployment to Afghanistan.
The BBC's Martin Patience in Kabul said British troops were increasingly being targeted by improvised explosive devices (IEDs) and that there were no signs of the Taleban-led insurgency abating.
Military chiefs spoke of the Afghan conflict in terms of years or even decades, he added.
Mr Brown described the four latest deaths in Afghanistan as "a tragic loss".
I know that the whole country is immensely proud of all those who served in our armed forces
Prime Minister Gordon Brown
"Our thoughts are with the families and friends of those brave men who died in the service of their country. We owe them a huge debt of gratitude. This is a tragic loss," he said.
"Fighting the Taleban in Afghanistan puts our armed forces in the front line against terrorism. I know that the whole country is immensely proud of all those who served in our armed forces and of what they do to protect each and every one of us."
Military spokeswoman Commander Paula Rowe said the deaths made for an "incredibly sad day for Task Force Helmand".
"The tragic deaths of these Royal Marines have come as a huge blow to us all," she said.
"Losing a loved one is one of the hardest things to go through. Our heartfelt sympathies go out to their families, friends and comrades at this terrible time."
Conservative leader David Cameron described the deaths were "a terrible reminder of the bravery of our troops who are serving our country".
"My thoughts go out to their families and friends at this tragic time," he added.
A British soldier serving with 20 Armoured Brigade has also died at Basra airport in Iraq after shooting himself. The MoD said there was no evidence to suggest anyone else was involved.
A week ago, Lance Corporal David Wilson died in similar circumstances at the same airbase.
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