The huge crater outside Islamabad's devastated Marriott Hotel in Islamabad is testament both to the growing reach of al-Qaeda and to the new-found confidence with which it operates.

The five-storey hotel is a favourite meeting place for businessmen, foreign diplomats and journalists. It is just a few blocks away from Parliament and the official residences of Prime Minister and President, at the heart of political power in Pakistan.

Intelligence officials say that the sophistication of the truck bombing suggested that it was the work of al-Qaeda. What is most alarming, however, is that al-Qaeda and other militant groups now seem to be operating in the capital and other major cities with impunity.

The hotel, which has been the target of smaller attacks in the past, is already surrounded by a high security zone. But security was particularly high yesterday because of a joint session of parliament addressed by Asif Ali Zardari, the newly elected President.

The explosion took place a couple of hours after that session was over, by which time Mr Zardari, his Prime Minister Yousuf Raza Gilani and other military and political leaders were attending a state dinner less than half a mile away.

Rehman Malik, Pakistan’s Interior Minister, blamed Tehrik Taleban Pakistan, an outlawed militant group operating from a lawless tribal region in the north. The group is said to be closely linked with al-Qaeda. Many other Pakistani militant groups have mutated into small cells after being banned and work as an extension of al-Qaeda.

It was not al-Qaeda's first involvement in an attack in the Pakistani capital. In June, a suicide car bomber killed at least six people near the Danish Embassy. A statement attributed to al-Qaeda took responsibility for that blast, believed to have targeted Denmark over the publication of cartoons depicting the Prophet Muhammad.

The latest attack came as Pakistani forces stepped up operation against the militants in Bajaur tribal region considered al-Qaeda's operational base. Many observers believe the Marriot attack could be a retaliation to the military offensive which has killed hundreds of militants. The fighting, the most intense since Pakistan allied itself with the United States in its War on Terror in 2001, has left hundreds of people dead.

Al-Qaeda and their allies among tribal militants had repeatedly threatened to escalate the conflict if that military operation was not stopped. Anti-American sentiments are also high in the country after surge in US predator attacks against suspected militant hideouts inside the Pakistani tribal region.

The weekend attack also raises serious question about security lapses. How did such a big truck, carrying an estimated 600kg of explosives, get so close to such an imporant terror target? A lot of questions will need answering.

Times Recommends

• Both sides blame US for worsening relations
• Stieg Larsson: losing the plot over his cash
• Odd creature paralysed by incompatible parts