A suicide bomber who was believed to be just 12 years old killed three Royal Marines in a bloody 24 hours for British forces in which five servicemen died in Afghanistan and Iraq.

The bomb in Helmand, detonated by the youngest known suicide attacker against Western forces in the country, was followed by a mine blast that killed another marine. In Basra, a soldier was shot dead.

Both the Afghan attacks were in the Sangin district, the first at about 10am. Marines from 45 Commando were on foot patrol passing through a village when they were approached by a boy pushing a wheelbarrow containing the bomb, which exploded. An hour later, a Jackal armoured vehicle struck a mine, killing the fourth commando.

The Jackal, a new type of heavily armoured vehicle, is designed to be mine-resistant and had been tested last year by the SAS. But this was the second fatality involving the vehicle, which was recently deployed in Afghanistan, with another marine killed on 12 November, providing evidence that the Taliban are adapting their tactics and using more potent devices.

The death in Basra of a member of 20th Armoured Brigade, defence sources say, appeared to be self-inflicted. It was the second case of suicide by a British soldier at the base in less than two weeks. On 4 December, Corporal David Wilson, 27, with 9 Regiment Army Air Corps, the father of an 11-week-old daughter, shot himself in the head.

Deaths and injuries from actual combat have declined significantly for UK forces in Iraq who are to start pulling out of the country from the beginning of March with all the 4,100 force, except a few hundred training the Iraqi army, leaving in three months.

British and other Western troops have increasingly been subjected to attacks by suicide bombers in Afghanistan. The grim milestone of UK fatalities reaching 100 came in the summer, when three members from the 2nd Battalion, the Parachute Regiment, were killed in such an attack in June.

Nato and Afghan government security forces also report the rising prevalence among enemy forces of young boys, mainly from madrassas, or religious schools, across the border in Pakistan.

The Independent has interviewed a 14-year-old boy, arrested on his way to a suicide mission, in the Afghan capital, Kabul. Shakirullah Yasin Ali said "If I had succeeded, I would be dead by now, I realise that. But those who were instructing me said that if I believed in serving God it was my duty to fight the foreigners. They said God would protect me when the time came."

Defence sources said that yesterday's attacks were being investigated. A senior officer said service personnel were obviously putting themselves at risk from suicide bombers by patrolling on foot, but that it was essential to mix with the local population and not alienate them by appearing only in heavily armoured convoys.