Interior Secretary Sayed Kamal Shah said a special unit of the Federal Investigation Agency, headed by a DIG, has been formed to investigate the blast. A suicide bomber on Friday tried to storm the hotel hours before a reception there by Indian High Commissioner Satyabrata Pal to mark the Republic Day, killing himself and a security guard and injuring seven others.

The investigators apparently were still clueless about the attack and had no idea why the bomber tried to enter the hotel from the side entrance, which is either used in emergency or by the hotel staff.

Questions like why they tried to enter several hours before the Republic Day reception and the real target of the attack were being probed, Shah said. Civil Aviation Authority spokesman Pervez George said, the stepped up security was a natural consequence of the attack on the hotel, although there was no specific threat to any of the airports.

All vehicles entering the airport premises were being searched thoroughly and all visitors and the airlines' staff were subject to body and baggage searches, media reports quoted Airport Security Force sources as saying adding that because of this, long queues could be seen outside the airports.

Officials have not ruled out that the blast could be the retaliation for a recent military attack on a madrassa in the Bajore tribal agency after which locals had threatened suicide bomb attacks in Islamabad.

Demolition of two unauthorised mosques in Islamabad during the past few days, which caused resentment among madrasa students were also not being ruled out.

The Indian High Commissioner Satyabrata Pal, who went ahead with the plans to hold a Republic Day reception at the same hotel after the blasts, said that the Pakistan government has not spoken to Indian officials on the incident.

"Nor do they need to. They are conducting their investigations," he told PTI in Islamabad.

"We treat it as a deplorable attack in Pakistan. We wish them well in their investigations to bring those who have perpetrated the blast to book," he said.

Both Pal and the Pakistan Minister for Education Javed Asharaff Qazi who attended the reception as chief guest departed from the practice of not making speeches at such receptions and asserted that such attacks would not deter the two countries from proceeding ahead with the peace process.

Qazi, who was former ISI Chief said, "we ourselves are victims of terrorism that is why we are fighting terrorism and continuing to do so until this scourge is eliminated from our country," he said.

"This peace process, Inshallah will go on till we have all people in the region living in peace and working together for prosperity of all people of this sub-continent," Qazi said.