The know-how and materials for manufacturing lethal improvised robots are easily available, according to experts gathered at a British military think-tank on Wednesday.

'Sooner or later, we are going to see a Cessna programmed to fly into a building,' said Rear Admiral Chris Parry, who formed the British Ministry of Defence's Development, Concepts and Doctrine Centre in 2005.

He said that small, remotely piloted planes or even converted model aircraft were 'ideal weapons' for terrorists because they were easy to build and could evade radar.

'They are cheap. They do not need as much motor power or fuel, and they are difficult to detect - about as difficult to detect as a blackbird,' he said.

Adm Parry's comments at the Royal United Services Institute were echoed by other speakers at the conference on robotics, among them computer scientist Richard Starkey.

'It is very easy to go to the Internet...or go down to the scrap yard and put a robot together,' Mr Starkey said. 'You do not need (it) to last long if you want to explode it among a civilian population.'

Both pointed to Hizbollah's deployment of pilotless aircraft against Israel in 2006.

Adm Parry alluded to the use of unmanned submarine-like vessels to ferry drugs across the Pacific and allegations that Hamas had explored fitting model aeroplanes with explosives.

Unmanned vehicles - from grim hunter-killer drones like the Predator to tiny toy-like explosives-disposal buggies like the 'Bombot' - are playing an increasingly important role in the US war effort in Iraq and elsewhere.

The Pentagon wants US$3.4 billion (S$4.7 billion) for this year to fund its unmanned aircraft programmes alone.

It is also increasingly interested in autonomous robots - machines which move around, scout and even kill without direct human oversight.

Experts at the robotic conference warned that developing such devices would create a morass of legal and ethical problems.

'How do we make sure they comply with the rules of engagement?' said Mr Chris Elliot, a fellow at Britain's Royal Society of Engineering.

If an autonomous robot attacked a civilian target, who would be held responsible, Mr Elliot asked. The manufacturer? The programmer? The operator?

'Can a robot commit a war crime?' he asked.

Mr Elliot predicted that moral concerns might keep lethal autonomous robots out of Western arsenals, but suggested that other countries or armed groups might not be so scrupulous.

'We are going to see autonomous systems deployed, but not by our side,' he warned. 'The technology that we are developing, we are first going to be seeing deployed against us.'

Underscoring the continuing threat from terrorists, Al-Qaeda second-in-command Ayman al-Zawahri, in a posting on the Internet on Wednesday, has vowed revenge for the killing of a top group commander.

'No chief of ours had died of a natural death, nor has our blood been spilled without a response,' he said in the video posted on an Islamist website, referring to the killing of Abu Laith al-Libi.

Libi, considered one of Osama bin Laden's top lieutenants in Afghanistan, died in a suspected US missile strike that killed up to 13 foreign militants in Pakistan's North Waziristan border area in late January.

ASSOCIATED PRESS, REUTERS