KABUL (Reuters) – Afghanistan's Taliban said on Sunday they would attempt to derail elections this month and warned Afghans to boycott the vote, the first explicit threat against the poll by the militants.

The September 18 parliamentary election is seen as a litmus test of stability in Afghanistan before President Barack Obama conducts a war strategy review in December that will examine the pace and scale of U.S. troop withdrawals from July 2011.
Despite the presence of almost 150,000 foreign troops, violence is at its worst across Afghanistan since the Taliban were ousted by U.S.-backed Afghan forces in late 2001.
""This (poll) is a foreign process for the sake of further occupation of Afghanistan and we are asking the Afghan nation to boycott it,"" Taliban spokesman Zabihullah Mujahid said.
""We are against it and will try with the best of our ability to block it. Our first targets will be the foreign forces and next the Afghan ones. So we are asking people to not take part,"" he told Reuters by telephone from an undisclosed location.
Security is a major concern ahead of the vote, with four candidates killed already in recent weeks, according to the United Nations and government officials.
Another candidate was wounded, and 10 of his campaign workers killed, in an air strike in northern Takhar province on Friday, Afghan President Hamid Karzai has said.
The NATO-led International Security Assistance Force (ISAF) is investigating the incident but maintains it killed a senior member of the al-Qaeda-linked Islamic Movement of Uzbekistan in the air strike.
The Islamists launched scores of small attacks against last year's presidential poll but failed to disrupt the process significantly.
Polling centers closed
According to Afghanistan's Independent Election Commission (IEC), 938 out of a planned 6,835 polling centers will not open on election day because of security fears.
""We are aware of the efforts of the IEC and the Afghan security forces to establish as many secure polling centers as possible, to ensure that voters are able to vote,"" the United Nations Assistance Mission in Afghanistan (UNAMA) said in a statement on Sunday.
""While it is a difficult decision not to open the polling centers in certain locations, we agree with the decision of the IEC to protect the security of voters, electoral workers and the secure and effective scrutiny of polling centers and voting procedures,"" it said.
Apart from security, graft and cronyism are also major concerns ahead of the vote after last year's fraud-marred presidential election, in which a third of votes for Karzai were thrown out as fake.
The issue of corruption frequently strains ties between Karzai and his Western backers, and the vote is also seen as a test of Karzai's credibility.
A U.N.-backed elections watchdog, which has been significantly weakened since last year's vote, has said dozens of candidates have been blocked from taking part because of their links to private militias.
On Sunday, the U.N.-backed Electoral Complaints Commission said it was concerned that some public officials were using their positions to help certain unidentified candidates.
It called on the government to ""adopt and implement measures to prevent such activities, which affect the impartiality and integrity of the elections.""
About 2,500 candidates are running for 249 seats in the Wolesi Jirga, or lower house of parliament, in Afghanistan's second parliamentary vote since the Taliban were ousted.
Foreign donors, mostly from the West, are covering the estimated $149 million cost of the poll.