An injection of 21,000 extra troops to Afghanistan this summer will provoke a jump in violence before the casualty rate falls, a top British commander has warned, as the deaths of two more British servicemen were announced.

Lieutenant-General Jim Dutton, Nato’s deputy commander in Afghanistan, said that he expects a repeat of the impact experienced by US Marines when they arrived in the south of the country last year: a sharp rise in skirmishes and casualties before a “marked reduction” three weeks later. The presidential election on August 20 is “where we’re planning for the worst case,” he told The Times.

Predictions of increased fighting enable military and political leaders to claim success by simply preserving the status quo, but consensus within the coalition and outside is that this summer will be particularly bloody.

Almost twice as many coalition soldiers died in the first three months of this year as in the same period in 2008, itself the bloodiest year since the conflict began in October 2001.

Highlighting the danger, a British soldier from 2nd Battalion The Mercian Regiment was killed yesterday in an explosion during an operation near Garmsir in Helmand province, southern Afghanistan. Hours earlier a member of the Royal Marine Armoured Support Group died of injuries sustained when a suspected roadside bomb hit his Viking armoured vehicle near Lashkar Gah, the capital of Helmand, a week ago. The families of the men have been informed.

The latest deaths bring the number of British military deaths in Afghanistan since 2001 to 163. It also makes May one of the bloodiest months of the campaign for Britain, with ten servicemen dead.

In other clashes, US and Afghan forces killed about 30 insurgents in an assault on a suspected foreign fighter camp yesterday morning. The camp, in the eastern province of Paktika, was near the border with Pakistan.

Security analysts blame the escalating conflict on infiltration of new areas by the Taleban, including provinces surrounding the capital, Kabul. They also say Taleban-led insurgents have an increasing number of fighters and guns, and are improving their tactics. This is the challenge facing General Stanley McChrystal, the newly installed top US and Nato commander in Afghanistan. A special forces commander, his use of intelligence is credited with helping to quell the insurgency in Iraq.

• About 400 Royal Marines received medals at a ceremony at their base outside Arbroath, Angus, yesterday. Members of 45 Commando also remembered nine Marines from the unit who died and others who were injured during a six-month tour to Afghanistan that ended last month.

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