An insurgent bomb killed eight Afghan security guards in restive southern Afghanistan on Tuesday, the government said, as a surge in attacks raised alarms just weeks ahead of elections.

The remote-controlled blast in the province of Helmand, where British troops have suffered heavy losses, ripped through two vehicles from a private Afghan security company, the interior ministry said.

"The bomb, which was planted by the enemies of Afghanistan ... struck two vehicles of a security company, killing eight guards and injuring four others," the ministry said in a statement.

The term "enemies of Afghanistan" is used by Afghan officials to refer to Taliban and other insurgents waging a bloody revolt that has stalled attempts to haul the country out of decades of war.

The attack came a day after Britain announced the end of a major offensive against the Taliban in Helmand, where the campaign focused on an area between provincial capital Lashkar Gar and its economic capital Gereshk.

Prime Minister Gordon Brown praised the "heroic" efforts of British forces for making "land secure for about 100,000 people" in Afghanistan, where more than 20 British troops have died so far this month.

Another roadside bomb killed a civilian construction worker and injured two others in the eastern province of Paktika on Monday, the Afghan interior ministry said separately.

Makeshift explosives are the weapon of choice for insurgents and cause most of the casualties inflicted on the roughly 90,000 foreign troops -- mainly US and British -- in Afghanistan to help defeat the Taliban-led uprising.

July has been the deadliest month on record for international forces in Afghanistan since the 2001 US-led invasion overthrew a Taliban regime.

The spike in violence highlights the threats faced by the war-torn country ahead of its second-ever presidential election set for August 20.

Ahead of the presidential and local elections, Afghanistan's Western allies have boosted their military presence with the United States deploying an extra 21,000 troops to Afghanistan this year.

Besides the British offensive, about 4,000 US Marines backed by Afghan troops have been hunting rebels in Helmand in a major assault to drive out Islamist militants from their strongholds and so allow Afghans to vote.

On Monday, the Afghan presidency announced that tribal elders had struck the first local truce with Taliban insurgents ahead of the polls, after three weeks of talks in the northwestern Badghis province.