JEDDAH, Dec 7: The gunmen who stormed the United States consulate in Jeddah on Monday had "scoped out" the building before their carefully planned attack, according to the U.S. envoy to Saudi Arabia.

Ambassador James Oberwetter also described a firefight at the gate before the militants slipped in on foot behind a diplomatic car entering the heavily fortified complex.

Four locally employed consulate staff and one guard were killed in the brazen daylight attack, the latest in an Al Qaeda campaign against Western and government targets. Four of the five attackers were killed after taking non-U.S. staff hostage.

But Mr Oberwetter said U.S. Marines stopped the gunmen entering the main chancery building where American staff were sheltering. "It's clear that some of these people have been looking at our procedures," Mr Oberwetter told reporters in Jeddah after he toured the site, which remained closed off on Tuesday.

"They clearly understood how our cars entered the compound and in my view had scoped it out." He said the gunmen tailed a consular car as it approached the sprawling complex in a busy area of Jeddah, swinging their car behind it just as it entered the consulate.

A barrier blocked the attackers' car, forcing the gunmen to get out. "The terrorists began to engage the local Saudi staff at the gate, who returned fire and a great firefight broke out."

"Regrettably the terrorists were not taken down there and were able to access the compound," Oberwetter said. Witnesses say once inside the compound the militants burnt the U.S. flag. Local staff working in outlying buildings told Reuters they were taken hostage and used as human shields. -Reuters