One of the most senior British army officers in Afghanistan has been killed by a roadside bomb in the southern province of Helmand, the defence ministry said on Thursday.

Lieutenant Colonel Rupert Thorneloe, 40, died with another British soldier when a bomb exploded under his armoured vehicle in a resupply convoy near the town of Lashkar Gah on Wednesday. The deaths were confirmed on the day thousands of American soldiers in Afghanistan launched the biggest offensive since U.S. President Barack Obama assumed office in January.

Thorneloe, the commanding officer of the 1st Battalion Welsh Guards, was the highest-ranking British soldier to be killed in Afghanistan since Britain joined the U.S.-led invasion in 2001.

Only about three British officers of Thorneloe's rank are typically deployed in Afghanistan at any one time, a defence ministry spokesman said.

The last British officer of a similar rank to die in combat was Wing Commander John Coxen, killed in a helicopter crash in Basra, southern Iraq, in May 2006, the spokesman said.

"The deaths...are a devastating blow to the Welsh Guards Battle Group and to the army as a whole," General Sir Richard Dannatt, the chief of the general staff, head of the British army, said in a statement.

The U.S. military said the new offensive, Operation Khanjar, or Strike of the Sword, was intended to seize most of the lower Helmand River valley, the heartland of the Taliban insurgency and the world's biggest opium poppy-producing region.

Violence in Afghanistan is at its highest since the invasion of 2001. Britain has about 8,300 soldiers fighting the Taliban, mainly in the south.

Thorneloe, who commanded more than 1,000 soldiers, and Trooper Joshua Hammond were the 17th and 18th British soldiers to be killed in Afghanistan since May. The total number of deaths for the British forces since 2001 is 142.