The extremist Taliban movement, which regularly uses suicide attacks in its campaign against the government, immediately claimed responsibility for the blast in Farah province's Delaram district.

The U.N. Special Rapporteur on extrajudicial executions Philip Alston said on Thursday that Taliban and other rebels had killed around 300 civilians in the past four months – roughly three-quarters in suicide attacks – while international forces had killed some 200 civilians during military operations.

Alston called the lack of open investigations into the hundreds of deaths reflected a "staggeringly high" level of complacency in the country.

In Thursday's suicide attack police were the target, Farah deputy governor Mohammad Younus Rasouli said, adding that four of the dead were police officers, targeted in a crowded bazaar.

"Twelve civilians have been killed and 22 others wounded. Four police have been killed and six others wounded," he told AFP.

Rasouli said the body of the bomber was destroyed. "But we have found pieces of women's dress, shoes and a burqa," he said, referring to the all-covering garment worn by most Afghan women which also hides the face.

The interior ministry in Kabul gave a lower death toll, saying seven civilians and five policemen were killed.

The police chief for southwestern Afghanistan, Ikramuddin Yawar, also said investigations had shown the bomber was wearing a burqa.

"The bomber wore a burqa, which indicates it might be a woman," he said.

However, Taliban spokesman Yousuf Ahmadi said a member of his organization – whom he identified as a man named Khalid – carried out the bombing, which he said killed a dozen police and was aimed a police commander.

Ahmadi's account could not be independently verified as the rebel spokesman has in the past made exaggerated or false claims.

President Hamid Karzai condemned the attack, calling it an act of a "coward," his office said in a statement. It also said the bomber wore a burqa and woman's dress.

If confirmed the bomber was female, it would be the first known suicide attack carried out by a woman. Male attackers have sometimes worn burqas as a disguise.

Juma Khan, an official in the provincial police department said women and children were also probably killed in the blast.

"The bombing was in a crowded bazaar. We think there might be women and children among the casualties," he said.

More than 70 people have been killed in six suicide attacks since early April, according to an AFP tally. The Taliban have claimed most the attacks.

The U.N. special rapporteur said that secret militias apparently controlled by foreign intelligence services also operate with impunity in the country, while police were also involved in unlawful killings.

"Afghanistan is enveloped in an armed conflict. But that does not mean that large numbers of avoidable killings of civilians must be tolerated," said Alston, a professor of law in New York who is independent of the United Nations.

"The level of complacency in response to these killings is staggeringly high."

Most of the 200 civilians reported killed by international troops, often working with Afghan forces, died in air strikes, Alston said, adding that he had seen no evidence of foreign soldiers violating the law or human rights.

However, the forces were "surprisingly opaque" about accounting for the incidents.

They had said figures for civilian casualties caused by troops were "either not available in Afghanistan ... or that they are secret and cannot be provided to me," Alston said.

He was also told the outcomes of investigations into individual soldiers suspected of unlawful killings were not tracked in Afghanistan so it was never known if anyone was held accountable.

Alston added it was "absolutely unacceptable" for foreign intelligence units to be "conducting dangerous raids that too often result in killings without anyone taking responsibility for them."

These units were outside of the government and Afghan structures and not much was known about them, he said.