Washington, May 3 (IANS) Director of the US Central Intelligence Agency (CIA) Leon Panetta warned Monday that Al Qaeda without Osama bin Laden is still dangerous."The terrorists almost certainly will attempt to avenge him, and we must - and will - remain vigilant and resolute," Panetta, who is going to be the next defence secretary, wrote in a letter to the agency's employees, Xinhua reported.

"But we have struck a heavy blow against the enemy, and we will not rest until every last one of them has been delivered to justice," he wrote.

US President Barack Obama Sunday night announced bin Laden, architect of 9/11 terrorist strikes in 2001 that killed over 3,000 people, was killed in a US raid in Pakistan. He said the death of the terrorist leader marks the "most significant achievement" to date in US effort to defeat Al Qaeda.

Shortly after this announcement, the State Department issued a worldwide travel alert warning of an "enhanced potential for anti-American violence given recent counterterrorism activity in Pakistan".

White House counterterrorism chief John Brennan Monday also warned of potential retaliatory attacks by Al Qaeda operatives or other terrorist groups. But he said there is not any credible information to warrant issuing a warning under the new terror alert system.