A band of up to 30 youths armed with makeshift weapons and some wearing masks attacked two riot police patrolling a housing project outside Paris in an apparent ambush that seriously injured one of the officers, police officials said Wednesday.

The attack Tuesday night raised the specter of the violence that racked poor suburbs last year. It came amid reports of growing suburban delinquency, with the prefect of the Essonne region, where the incident occurred, saying that youth gangs were engaged in "acts of war."

One of the officers, who sustained a double skull fracture, was undergoing surgery Wednesday at a Paris hospital, a police official said. The other was released after treatment from a hospital in Evry.

The two were attacked by 20 to 30 youths while patrolling in an unmarked car around a housing project known as a trouble spot in the town of Corbeil-Essonnes, police said.

The youths stoned the car, encircled the officers when they got out, then attacked, according to various accounts.

"The youths were waiting in ambush in the woods and jumped out, their faces masked, to strike them with fists and pieces of wood," Joaquin Masanet, head of the UNSA-Police union, said in an interview. "The two officers had fallen into a trap and they (youths) were hitting them to kill."

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Police were called in to disperse the gang but no arrests were made.

"We'll find them one by one ...," Interior Minister Nicolas Sarkozy vowed Wednesday. "Not just (here) but in every place in France where law enforcement officers are attacked."

He promised "extremely severe" punishment.

Prefect Gerard Moisselin, the highest state official in the Essonne region, said an investigation was under way, but culprits like those who carried out the attack "disappear into buildings."

Such bands "show great aggressiveness toward police," Moisselin said on the LCI television station.

"We're dealing with veritable acts of war against police led by small groups," he said.

French authorities have kept a watchful eye on the suburbs surrounding big cities where most low-income housing projects are located, fearful of a new explosion of violence like the riots that erupted in poor housing projects nearly a year ago and continued for three weeks.

However, in parts of the Paris region, at least, there are signs that juvenile delinquency has only climbed.

Jean-Francois Cordet, prefect of the Seine-Saint-Denis region north of Paris -- where the riots began -- sent an angry letter to Sarkozy in June complaining about a 14 percent increase in physical violence, a nearly 23 percent increase in theft with violence and the sense of impunity of juvenile delinquents. The daily Le Monde published excerpts of the letter.

Prime Minister Dominique de Villepin said authorities would "draw the lessons" from the incident in Corbeil-Essonne, instituting "appropriate techniques to better anticipate risks" to police.