BAGHDAD, Iraq — U.S. jets pounded parts of Fallujah today, targeting insurgents in a city where American forces were said to be gearing up for a major offensive.

Al-Jazeera television broadcast a threat by an unspecified armed group to strike oil installations and government buildings if Americans launch an all-out assault on Fallujah. The report was accompanied by a videotape showing about 20 armed men brandishing various weapons including a truck-mounted machine gun.

Gunmen kidnapped a Lebanese-American businessman — the second U.S. citizen seized this week in Baghdad — and videotape Wednesday showed the beheadings of three Iraqi National Guardsmen and an Iraqi officer.

Early today, U.S. aircraft fired on several barricaded rebel positions in northeast and southeastern Fallujah, the military said.

U.S. and rebel forces also clashed overnight on the southeastern outskirts of the city after insurgents fired a rocket-propelled grenade at Marines. Two insurgents were killed while no U.S. casualties were reported, said Lt. Nathan Braden, of 1st Marine Division. Hospital officials in Fallujah reported three civilians were injured in the overnight shelling.

U.S. forces are preparing for a major offensive in Fallujah, west of Baghdad, and other Sunni militant strongholds in hopes of curbing the insurgency ahead of January’s election.

An Iraqi National Guard patrol was hit today by a car bomb in Iskandariyah, an insurgent hot spot 30 miles south of Baghdad, killing at least three people, Iraqi officials said.Iskandiriyah Hospital officials said three people had been killed and a total of 15 others injured, including the guardsmen.Gunmen kidnapped a Lebanese-American businessman — the second U.S. citizen seized this week in Baghdad — and a videotape aired Wednesday showing the beheadings of three Iraqi National Guardsmen and an Iraqi officer.On Wednesday, a U.S. soldier was killed and another wounded in a roadside bombing 12 miles south of the capital. A suicide driver detonated his vehicle at a checkpoint near Baghdad airport, injuring nine Iraqis and forcing U.S. troops to close the main route into the city for hours.Gunmen killed a senior Oil Ministry official, Hussein Ali al-Fattal, after he left his house in the Yarmouk district of western Baghdad, police said. Al-Fattal was the general manager of a state-owned company that distributes petroleum byproducts.The violence served as a grim reminder of Iraq’s rapidly deteriorating security situation, which President Bush must address now that he has won his long electoral contest against Sen. John Kerry.Radim Sadeq, an American of Lebanese origin who worked for a mobile phone company, was grabbed about midnight Tuesday when he answered the door of his home in Baghdad’s upscale Mansour neighborhood, officials said. No group claimed responsibility.It was the second abduction this week in Mansour, where many foreign companies are based. On Monday, gunmen stormed the two-story compound of a Saudi company, abducting six people, including an unidentified American, a Nepalese, a Filipino and three Iraqis, two of whom were later released. No claim has been made for the kidnappings.More than 170 foreigners have been kidnapped and more than 30 of them killed in Iraq since Saddam Hussein’s regime fell in April last year. At least six of the foreigners were beheaded by followers of Abu Musab al-Zarqawi, a Jordanian militant who has sworn allegiance to al-Qaida.