By Lynne O'Donnell

KABUL, AFGHANISTAN - A suicide bomb that shook Kabul on Saturday has brought the fight against Islamist insurgents to the front door of countries promising safety and security to the people of Afghanistan, analysts said.

Coming just days before August 20 elections, the attack was viewed as an audacious signal from the Taliban to the Afghan government and its international supporters that they can strike anywhere, any time.

The car bomb, claimed by the Taliban, exploded at the entrance to the Kabul headquarters of the international military organisation, NATO, that has more than 100,000 troops in Afghanistan trying to wipe out the Islamist rebellion.

The bombing 'was a warning that the Taliban can attack any time,' said author and analyst Waheed Mujda, who worked for the fanatics' administration during their time in power from 1996-2001.

'The tactics they use make them very difficult to stop,' he said.

The explosion - a 500 kilogramme (1,100 pound) car bomb, a Taliban spokesman said - came five days before the country's second presidential election, which has been shrouded in fears of violence and suicide attacks.

Those fears - vindicated by the attack - could see large numbers of voters, especially in vulnerable rural areas, stay away from the polls, undermining the credibility of the election.

'More than the attack itself, it is important to focus on the place,' said Haroun Mir, an analyst with the Institute of Research and Police Studies.

The explosion was outside the gate of NATO's Kabul headquarters, near the US, Spanish and Italian embassies, and the biggest US military base in the city.

'If they are able to attack a well-protected area like that, then to attack voting centres in Kabul or any other major city will be much easier,' said Mir.

'The other Afghan cities are much less secure, and it's worse in the countryside.'

While the Taliban has vowed to disrupt the elections, analysts said the Saturday attack also stabbed at the heart of the international military effort that NATO says aims to bring security and democracy to Afghanistan.

It was the first attack on the headquarters of NATO's International Security Assistance Force (ISAF) since it was set up in Kabul in 2002, and a fearless breach of one of the most heavily fortified areas of the city.

While the Taliban have said they will not directly attack polling stations, and some local commanders are said to have struck peace deals for election day, senior insurgent leaders have said no such deals exist.

Mir said the Kabul bombing - which killed seven Afghans and injured almost 100 people - was proof of Taliban intent and capability.

'They didn't kill any foreigners, they killed only Afghans, so when they say they won't attack civilians on voting day they're totally lying,' he said. But beyond the election - which the Afghan government and its outside supporters had hoped would showcase the country's progress towards democracy - the attack also aimed to demonstrate the futility of the ISAF military effort.

NATO troop numbers have been boosted in recent months as soldiers go on the offensive in Taliban strongholds in the north and east, part of a new counter-insurgency strategy led by US General Stanley McChrystal.

As ISAF and US forces push harder against Taliban resistance, foreign force casualties have reached record highs in recent months, with almost 80 so far in August.

The escalating death toll has led to questions in Washington and London about the wisdom of staying in Afghanistan, and concerns that as public opinion turns against the war, some NATO partners may pull back their troops.

US President Barack Obama has already ordered an additional 21,000 troops to Afghanistan and the number of US forces is set to reach 68,000 before the end of the year.

McChrystal is due to deliver an assessment of the new strategy to Obama next month but US Defence Secretary Robert Gates last week expressed concerns about an imbalance in the ISAF force.

Insurgent disruptions to the election could play into those fears, said Jandad Spinghar of the Free and Fair Election Foundation of Afghanistan.

'The Taliban were sending a message that they are serious about disrupting the election process and it is probable that many people, especially women, will take it as a warning to stay away,' he said.

'Beyond that, they know that the polling day is a priority for both the Afghan government and the international community, and that they have promised they will provide a secure environment for voters.

'Now the Taliban are saying they have enough power to stop the international community from bringing security and in doing so they are showing that they cannot be stopped.'