The early morning blast at the US Protection and Investigation (USPI) compound in the troubled southern city of Kandahar was the deadliest in a recent string of similar attacks blamed on the ousted Taliban regime.

"USPI announces a suicide bomber attacked their Kandahar operation early Wednesday. Two Americans and three Afghans were killed and many other persons were injured," a statement issued by the company said, adding all the dead worked for the firm.

The injured included both civilians and USPI employees, the company statement said, without providing details.

A regional chief of USPI was one of those who died in the attack, according to a USPI employee at the scene, Mohammad Afzal, but the firm's office in the capital Kabul refused to comment further.

Two Afghan interpreters linked to the firm were also killed, bringing the death toll to five Afghans and two foreigners, Interior Ministry spokesman Zemarai Bashari told AFP. He earlier said three foreigners had died.

A doctor at the Mirwais Hospital in Kandahar, speaking on condition of anonymity, also confirmed that it had received the bodies of five Afghans.

Police said the bomber approached on foot and detonated explosives strapped to his body outside the heavily guarded office, blowing him to pieces. No one has claimed responsibility for the attack.

Texas-based USPI is one of the biggest security firms working in Afghanistan, where it has operated since 2002. It employs 45 expatriates and hundreds of Afghans, some of whom are former members of private militias.

The attack was the ninth suicide bombing in Afghanistan since November 25. Most of these have been in Kandahar, including one that killed two Canadian soldiers on November 27.

Two more Canadian troops and four Afghans were wounded in a suicide car bombing in Kandahar on Tuesday.

Private security firms protecting foreign businesses, military bases and a host of organisations in lawless Afghanistan have been targeted by militants on a number of occasions.

In July, Taliban gunmen ambushed a USPI convoy in the western province of Herat, killing two local security guards.

Violence in Afghanistan has left more than 3,700 people dead this year, four times the toll for 2005, according to an official report.

Most have been insurgents but the figure includes about 1,000 civilians, plus Afghan soldiers and police.