Six British soldiers have been killed by an explosion in southern Afghan province of Helmand, according to the UK's defence ministry, in the deadliest attack on the country's forces in over five years.

A separate explosion in a crowded market in neighbouring Kandahar province killed four civilians and wounded nine others, provincial officials said.

The soldiers were on patrol in Helmand's Gereshk district on Tuesday when their armoured vehicle was caught by the blast, the ministry said in a statement on Wednesday.

"One of the Warriors [armoured vehicle] suffered catastrophic damage in the course of that explosion," Brigadier Patrick Sanders, commander of Task Force Helmand, told journalists in the provincial capital Lashkar Gah.

Mohammad Ismail Hotak, a senior Afghan police official in Helmand, said the explosion was caused by a roadside bomb.

However, Al Jazeera's Bernard Smith, reporting from the capital Kabul, said there was confusion about the cause of the blast.

The soldiers were out of their armoured vehicle and had gone just about 500 metres outside the base when there was an enormous explosion, he said.

"We're not sure if this was a roadside bomb or something else," said Smith.

Our correspondent said that the Afghan authorities he had spoken to suggested that because the explosion took place so close to the base, which is monitored by close circuit television cameras, it would have been very difficult for anyone to plant a homemade bomb.

Increase in fatalities

David Cameron, the British prime minister, told the British broadcaster BBC: "This is a desperately sad day for our country and desperately sad, of course, for the families concerned."

Foreign combat troops are due to leave Afghanistan at the end of 2014, but doubts persist about the ability of Afghan soldiers and police to control the country.

Most of the UK's 9,500 soldiers currently serving with the NATO-led International Security Assistance Force (ISAF) are based in Helmand.

The recent deaths bring the total number of British forces who have died while serving in Afghanistan to more than 400 since the US-led invasion to topple the Taliban in October 2001. The incident is also the deadliest attack on British troops since 2006.

So far this year, more than 60 NATO troops have been killed in Afghanistan, including 38 from the US and 10 from Britain.

Source: Al Jazeera and agencies