GAS canisters are being left inside burning stolen cars in a deliberate attempt to explode them as firefighters arrive, it was revealed today.

Now fire chiefs are warning it is only a mattter of time before a member of their crews is maimed or killed by the potentially deadly booby traps - sometimes set by children as young as ten.

In the most serious incident, gas canisters were left inside four stolen cars near a railway line in the Duffryn area of Newport, at Pencarn Lane. They were also left inside burning cars in Tredegar and Varteg, near Blaenavon, over the last 12 months.

Dick Pearson, the FBU's regional secretary, said firefighters' lives were being put at risk by gas canisters - such as LPG or aerosols - being left inside burning cars. There were 16 attacks against firefighters throughout Gwent between April 2004 and March 2005. "There s a trend in South Wales - in Newport and Cardiff - of cars being stolen, set alight and set up with gas canisters to explode as ambush," said Mr Pearson.

"The problem is being exacerbated by the fact that it could end up with a firefighter being killed in action."

Caldicot firefighter and FBU chairman for South Wales, Nigel Williamson, said the canisters were being left as "booby traps".

He said: "These canisters can cause damage which is disproportionate to their size and can be like a grenade going off. The problem with these canisters is that when a fire is involved, they become quite unpredictable.

"The people involved in attacks are usually young adults down to 11 or ten years old - maybe even younger than that. They seem to have a cartoon reaction to what they are doing and think that it's funny, and don't seem to realise that somebody could actually be seriously hurt or that there is the potential for a fatality.

"They don't seem to realise they are damaging their own communities." Earlier this week firefighters throughout the UK called for a national strategy to deal with the rising number of attacks on fire crews.

A Fire Brigades Union study found that fires are being started deliberately or false emergency calls made to lure firefighters into "an ambush", where they are abused and attacked.

In Wales and England in the nine-month period to the end of January, 393 attacks