Concerns over security and transparency in Afghanistan's parliamentary election grew on Friday after another candidate was attacked and a German observer sought to temper expectations of the poll.

The September 18 vote is seen as a litmus test for stability, as well as a test of the credibility of Afghan President Hamid Karzai after a fraud-marred presidential poll last year.

Violence across Afghanistan is at its worst since the Taliban were ousted by U.S.-backed Afghan forces in late 2001, raising fears that poor security will lead to low voter turn-outs, especially in the ethnic Pashtun belt in the south and east, where the Taliban-led insurgency is strongest.

In the latest attack, a candidate was wounded in a grenade attack in Ghazni city, southwest of Kabul, on Thursday, the NATO-led International Security Assistance Force (ISAF) said.

The candidate, Moulana Abdul Rahman, and an Afghan civilian were wounded when the grenade was thrown in the governor's compound in Ghazni.

The attack came on the same day that Karzai strongly condemned an apparent air strike by NATO-led forces that he said killed 10 campaign workers for another candidate in northern Takhar province. The candidate was also wounded in the strike.

At least four candidates have been killed and at least 15 campaign workers and supporters have been wounded in recent weeks, raising doubts about whether Afghan forces will be able to secure the poll adequately despite the presence of almost 150,000 foreign troops in Afghanistan.

Afghanistan's Independent Election Commission has said more than 930 polling centres, out of a total of 6,835, would not open because of security concerns.

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.

More than a third of ballots cast for Karzai in last year's presidential vote were thrown out as fake and some observers have already warned that this year's vote also won't be trouble-free.

It will be difficult, Germany's special representative for Afghanistan, Michael Steiner, told reporters in Berlin.

We shouldn't fool ourselves -- standards will not be on a level with Europe. It's more about reinforcing the mechanism.

Taliban insurgents, who carried more than 130 attacks against polling centres and others targets during the 2009 presidential poll, have so far made no direct threats against the parliamentary vote.

(Additional reporting by Brian Rohan in BERLIN; Editing by Paul Tait and Alex Richardson)