A roadside bomb killed 14 mourners travelling to Basra on a bus today in the deadliest in a wave of attacks across Iraq, which further highlighted the fragility of recent security gains.

A US military spokesman said that the unrest could be an attempt by al-Qaeda to divert attention from a renewed push by US and Iraqi forces in and around the northern city of Mosul, seen as the last urban stronghold of the extremist group.

He was speaking after eight American soldiers were killed in two separate attacks yesterday – the bloodiest day for the military in half a year. A roadside bomb in the restive province of Diyala left the three soldiers and their interpreter dead. A fourth serviceman was also wounded. The five soldiers were killed and three wounded when a suicide bomber blew himself up in the middle of a foot patrol in the upmarket Mansour district of west Baghdad. Two civilians also died in the explosion, which occurred outside a clothes shop.

The bus explosion occurred as a US convoy passed in the opposite direction on a highway between the city of Nassiriyah and Basra, according to a British military spokesman who had spoken to an Iraqi official at the site of the attack.

Eighteen people were also injured, including women and children.

Major Tom Holloway said: “Iraqi highway police are controlling the scene.”

The passengers had been returning from mourning rites for a dead relative in Najaf, a holy Shia city south of Baghdad.

Rear Admiral Greg Smith, the US military spokesman, said a spate of violence in the past few days could be a sign that al-Qaeda is trying to hit back. “Some of these recent attacks may be al-Qaeda's attempt to divert us from efforts in Mosul and northern Iraq where we have been pursuing them; a tactic we've seen before from AQI (al-Qaeda in Iraq),” he said. “This is a tough fight and although al-Qaeda has been hurt, it remains lethal and despite the gains of the past eight months, we have additional work to do.”

US and Iraqi forces are pushing north into Nineveh province, with a particular focus on the provincial capital of Mosul, in a bid to flush out al-Qaeda and other pockets of resistance that retain a presence in the area after being driven from Baghdad and western Anbar province as well as the southern belts.

Police said that four Iraqi policemen, four gunmen and a civilian were killed today in an attack on a checkpoint in Mosul.

Rear Admiral Smith reiterated that plans remained on track to reduce the number of American soldiers by more than 20,000 by the end on July, with 2,000 troops due to rotate out of Baghdad in the coming weeks.

With the casualty toll approaching the 4,000-mark, however, the topic of future withdrawals after July looks set to gain even greater attention in the US Presidential elections.

John McCain, the Republican presidential candidate, has been a strong supporter of President Bush’s troop surge policy in Iraq. In contrast both contenders competing for the Democratic nomination, Hillary Clinton and Barrack Obama, favour a speedy exit.

Senator McCain is due to travel to the Middle East and Europe on a 10-day trip in mid-March, which could include a stop-off in Iraq.

A 60 per cent drop in violence across Iraq since last summer has been attributed to the surge of 30,000 extra American soldiers as well as a ceasefire by the powerful Shia al-Mahdi Army.

A decision by Sunni Arab tribes to turn against al-Qaeda and side with US forces is another pivotal factor. Forming civilian security groups, these men work in their own neighbourhoods under the payroll of the US military, but have become a favourite target for extremists wanting to undermine the partnership.

In the latest attack, a suicide car bomber hit a checkpoint in Dhuluiya, north of Baghdad, today killing five people, including three members of such a neighbourhood security unit.

At least another 10 people died in clashes between security forces and the Mehdi Army militia, despite the ceasefire, in the city of Kut, southeast of the capital.

Moqtada al-Sadr, the influential Shia cleric, renewed the half-year ceasefire last month, but issued a statement at the weekend authorising his militia to act in self-defence if they were attacked. The cause of the clashes in Kut was unclear.

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