Afghanistan's 17 million voters were set to choose a president for only the second time in history with security forces on high alert Wednesday against a threatened Taliban onslaught. Ahead of Thursday's presidential and provincial council polls, authorities have been battling to reassure voters that it will be safe to visit polling centres as US and allied forces step up their anti-insurgency offensive. But a suicide bomber and rocket strikes on the capital on Tuesday escalated tensions in the final run-up to the election, which western-backed President Hamid Karzai hopes to win by a big enough margin to avert a run-off vote. "It (security) is of concern but to get free of this situation, we must cast our vote," said a 25-year-old Kabul woman employed by an international company who gave her name only as Massouda. Others said the risks were not worth it. "I will not let my family to go and vote in such a bad situation," said Abdul Qadir, a 40-year-old cigarette seller in Kabul where Tuesday's suicide blast killed a NATO soldier and nine Afghans, two of them UN staffers. "We should elect our president -- one that the entire nation will be happy with," said Haji Mohammad from the nearby province of Logar. But he added: "If election day is bad, I will not let the family vote." The government appealed for an Afghan and international media blackout on reporting any attacks Thursday "in view of the need to ensure the wide participation of the Afghan people". Taliban threats to sabotage the election have raised concerns that voter turnout could be low, compromising the legitimacy of the 223-million-dollar exercise in democracy. Attacks across Afghanistan Tuesday by the Islamist militia killed up to 15 people in total, with one rocket slamming into Karzai's presidential compound in the heart of Kabul. Thursday marks only Afghanistan's second direct presidential election, in a crucial test of a system installed after the Taliban were ejected from power in a US-led invasion in late 2001, following the September 11 attacks. It is a difficult process in a country where more than 70 percent of people are illiterate and tied into a tribal and religious system of fierce allegiances. While Karzai has been tipped to hold on to power, an energetic campaign by ex-foreign minister Abdullah Abdullah has boosted the chance of a run-off that election authorities say would take place six weeks down the line. Claims of vote-buying and biased use of government resources have added to concerns about the credibility of the election, along with rampant corruption and Karzai's reliance on warlords who stand accused of rights abuses. Despite billions of dollars of Western aid, most Afghans still lack electricity, roads are bad, jobs are scarce and graft rife. A team of visiting US senators, however, expressed optimism Tuesday about the evolution of democracy. "It is very gratifying, even thrilling, to see a vibrant election campaign occurring here in Afghanistan," independent Senator Joe Lieberman told reporters. "A lot of people who have been disenfranchised here for too long are clearly actively involved in determining the future of this proud people, proud country," he said. But in the hours before voting centres were due to open at 7:00 am (0230 GMT) Thursday, it was unclear how many sites would be operational with election officials pressing for more guarantees from beleaguered security forces. The number could lie anywhere between the 6,200 polling stations of the 2005 parliamentary elections, and an ideal of nearly 7,000, election officers said. Security officials have said at least eight districts out of 365 nationwide remain outside state control and up to 12 percent of polling centres may not open. But the NATO-led International Security Assistance Force, a crucial pillar of security in volatile Afghanistan, said Tuesday that less than one percent of polling stations were at risk of attack. About 300,000 Afghan and foreign troops -- every man available, officials say -- will be deployed to guard voting sites.