A top U.S. counter-terrorism official on a visit to Indonesia has said new tactics by terrorist groups will present the international community with greater challenges, warning the terrorists may resort to biological or chemical weapons as their knowledge and sophistication grow.

In Jakarta Monday, the U.S. State Department's coordinator for counter-terrorism, Henry Crumpton, said there was a growing threat that terrorists could use weapons of mass destruction.

"My concern is that as knowledge grows, as knowledge is diffused and people can learn more about toxins, more about biological and chemical weapons, that they will use them," said Mr. Crumpton. "Now the question is their capability. But I think in time some groups will gain that capability and you should be worried, you should be frightened. I am."

Mr. Crumpton was speaking to journalists shortly after meeting with Indonesian officials to discuss counter-terrorism efforts and information sharing.

Mr. Crumpton says Indonesia and the United Sates have been collaborating well on the fight against terrorism, but this cooperation needs to be boosted to counter the threat.

"This is going to require even greater international cooperation than we have now," he said. "One parallel is you look at avian influenza, how that is going to affect us all. Well if you match that kind of global threat with the intent of some of these terrorist organizations, then it is going to be a major challenge, a growing challenge for all of us."

Indonesia has been hit by a number of terrorist attacks during the past three years that the authorities have blamed on the al-Qaida linked regional terrorist group Jemaah Islamiyah.

The group is blamed for the 2002 bombings on the holiday island of Bali that claimed 202 lives, many of them foreign tourists, and for the suspected suicide bombings there earlier this month that claimed 20 lives.

While Indonesia has won praise for arresting and jailing more than 30 people for the first Bali bombings, sentencing three men to death, it has also been criticized for failing to outlaw JI as an organization.

Mr. Crumpton says banning the group may be an effective tool against the shadowy group.

"I certainly think that JI, given the crimes some of its members have committed, needs to be a focus of intense policy and legal attention," continued Mr. Crumpton. "And if that would include the banning of that group or some splinter of that group maybe that would be worthwhile. Would it be effective or not? I am not sure, but I think it might be."

The group is also thought to be responsible for the 2003 bombing of the Marriott Hotel in Jakarta that claimed 12 lives and the bombing outside the Australian embassy in Jakarta in 2004 that killed 10 people.

Mr. Crumpton is visiting Southeast Asia just two months after being appointed. He visited Malaysia on Monday and after Indonesia will travel to Singapore and the Philippines.