Afghanistan's parliamentary elections are being threatened by Taliban violence and intimidation as the government fails to protect candidates, especially women, a human rights watchdog said Thursday.

Afghans are set to vote in the country's second parliamentary polls on September 18, when around 2,500 candidates will contest the 249 seats in the lower house of parliament, the Wolesi Jirga.

The Taliban, who have been waging an intensifying insurgency for almost nine years, have said anyone associated with the poll is a target and have so far been blamed for the killing of at least three candidates.

Many others working on the elections have been attacked and kidnapped, with women candidates said to be the most vulnerable to intimidation and threats.

In a statement condemning the pre-election violence, New York-based Human Rights Watch (HRW) said: "Candidates -- as well as their staff members and election officials -- face assassinations, kidnappings and intimidation by insurgents as well as by rival candidates.

"Women candidates are facing the highest level of intimidation."

US Secretary of State Hillary Clinton joined calls for the Kabul government to provide adequate security for all participants in the poll, including voters.

Rachel Reid, HRW's Afghanistan researcher, was quoted as saying that the Taliban attacks and the "broad lack of confidence" in the Afghan government's ability to carry out a safe election, "threatens its validity".

"Insurgent violence, particularly against women candidates, was inevitable, but the government’s weak response was not," she said.

"While some candidates have complained to Human Rights Watch about the government’s lack of provisions for protecting candidates, others have not requested help or turned it down, citing a lack of confidence in the Afghan security forces," HRW said.

The Taliban threats and the deteriorating security situation follow last year's presidential poll, which was marred by extensive fraud, most of it found to be in favour of President Hamid Karzai.

The Taliban killed and injured hundreds of people leading up to and on election day last year, declaring the poll a tool of foreign occupiers, mainly the United States which is leading anti-insurgency efforts.

The United States and NATO have 150,000 troops in Afghanistan, some of which will be deployed to help secure polling centres.

Afghan election authorities have said that security can be provided for 5,987 polling centres, out of a total of 6,835.

Most of those that will remain closed are in Nangahar province, on the eastern border with Pakistan, according to observer Democracy International.

The Independent Election Commission (IEC), which oversees the poll, has said the polling stations will remain shut because security cannot be guaranteed.

Candidates and their supporters have been bombed, kidnapped, shot and -- in one case in troubled Ghazni province south of Kabul -- beheaded.

Last month five people working for candidate Fawzya Gailani in eastern Herat were kidnapped and killed in what remains the most shocking attack on election workers since campaigning began in late June.

That attack sent a message to women candidates that "not just in Herat but in every other province it may not be safe for you to campaign," said Democracy International's Jed Ober.

Most threats reported to the Free and Fair Election Foundation of Afghanistan were against women, HRW said, citing "at least 40 incidents of threatening letters or phone calls in 10 provinces".

Electoral law mandates 68 seats, or 28 percent, must go to women.

The European Union is spending 550,000 euros on television and radio advertisements for women candidates nationwide, EU envoy Vygaudas Usackas said.

Parliamentarian Daud Sultanzoi said Afghan security forces were ill-equipped to guarantee security for the elections.

"The security apparatus of this country are trying very hard but they are not equipped properly and the coalition and Afghan forces are not coordinated very well," he told Al Jazeera television.

"Sometimes in some areas action is missing. Security threatens it (the election). People's confidence is at stake," he said.