If the tens of thousands of Western expatriates who find themselves directly in Al-Qaeda's sights decided to leave Saudi Arabia en masse in light of an upsurge in terror attacks against them, their departure would have a devastating impact on the Islamic kingdom's key economic sectors.

That is what the militants hope to achieve as they notch up a gear in their campaign to overthrow the Al-Saud ruling family, and there are early signs that they may be on their way to achieving their goal.

Travel agencies in the Eastern Province, scene of the hostage drama in Khobar on May 29, in which 22 people were killed, say big joint ventures and multinational groups have made recent mass bookings for their American and Europe an executives.

There are even unconfirmed reports in the government-guided Saudi media of mass resignations from the state-owned energy giant Saudi Aramco, where Americans make up the bulk of the more than 10,000 Westerners whose expertise the kingdom still largely relies on to run its most vital economic sector.

Islamic militants have said that they gunned down one American and abducted another on Saturday in Riyadh, the latest in a string of well-planned attacks that have singled out Westerners and other non-Muslims working in the oil and arms industries.

Paul Johnson, an engineer working on a sophisticated targeting system for attack helicopters in Saudi Arabia, was kidnapped by an Al-Qaeda gang that said it also killed Kenneth Scroggs, who worked for Saudi-based Advanced Electronics Co.