Afghanistan's top security officials warned they were unable to guarantee the security of this week's elections, despite efforts to buy off Taliban insurgents.

Abdul Rahim Wardak, the defence minister, said yesterday that the security forces faced a formidable challenge in protecting Thursday's presidential and provincial elections, but the nation was resolved to go to the polls regardless of the threat from Islamist militants.

“To be 100 per cent sure that you can defend against the type of enemy you are dealing with, which has no respect for human dignity, will be very difficult,” Mr Wardak said.

“There is no doubt that the security forces of the International Security Assistance Forces and those of Afghanistan are not enough to secure this much area.”

The election is proving a much tighter race than had been expected with Abdullah Abdullah, a former foreign minister, staging a real challenge to President Hamid Karzai, who has been leading in the polls.

Mr Wardak's comments came after a suicide bomb attack outside the Nato headquarters in Kabul on Saturday intended to undermine confidence in the safety of about 6,500 polling stations across the country.

The attack struck Kabul's diplomatic enclave in the heart of the city. It was aimed at one of the most high-profile international targets – the headquarters of ISAF – as people reported for work. The blast, the first suicide bomb attack in the capital for eight months, killed eight people and injured close to 100.

The attack shattered an eerie peace in Kabul, that had held in the run-up to the election. The calm had led many to believe that Taliban insurgents had decided to spare the election a wave of a violence.