AN NASIRIYAH, Iraq (AP) — Coalition forces encountered the fiercest resistance from Iraqi troops yet near this southern Iraqi city, with US troops suffering a double ambush that left about 10 US Marines dead and another 12 soldiers missing, US Central Command said.

Meantime, devastating new explosions rocked downtown Baghdad on Monday morning as allied warplanes buzzed the city, a day after Iraqi police and security agents searched the banks of the Tigris River for a possibly downed coalition fighter pilot.

The new air raids were the heaviest since Friday night. Low-flying aircraft swooped overhead just before every huge explosion.

Behind the blasts, a mosque’s loudspeakers blared “Allahu Akbar” meaning “Thanks be to God”, perhaps meant as a morale boost, since it was too early for the daily prayer call.

The Iraqi military claimed 25 US troops were killed yesterday in the action near An Nasiriyah, a strategic city on the Euphrates River 320 kilometres (200 miles) south of Baghdad.

“It was a tough day of fighting for the coalition,” US Brigadier General Vincent Brooks told a briefing at the Central Command’s headquarters in the Gulf state of Qatar.

Meanwhile, other US-led ground forces made a sweeping, flanking advance across hundreds of miles of rough desert terrain to come within a day’s march of Baghdad. And allied troops were still trying to mop up resistance at Iraq’s main Gulf port of Umm Qasr.

Speaking at the Qatar briefing, US Army Lieutenant General John Abizaid said US Marines were ambushed near An Nasiriyah by Iraqi forces. About 10 Marines were killed.

In a second incident, 12 soldiers were missing after a six-vehicle supply convoy was attacked near the city, apparently after the driver took a wrong turn, Brooks said.

Abizaid called the fighting around the town the “sharpest engagement of the war thus far”. Besides US troops killed, he said some troops were wounded but gave no number.

Iraqi officials continued to claim they were holding their own on the battlefield. Defence Minister Sultan Hashem Ahmed disputed US claims that An Nasiriyah was seized, saying Iraqi forces repelled an attack from more than one direction. He said the Iraqis destroyed four tanks.

US Marines of the 1st Marine Division said late yesterday they expected to move around An Nasiriyah rather than through the city on their march to Baghdad to topple Iraqi leader Saddam Hussein.

The Marines near the city built a combat bridge across the Euphrates to use beside a captured civilian bridge. Troops, equipment and supplies were being moved over both of them.

Throughout the afternoon and evening, Apache attack helicopters flew over An Nasiriyah and over convoys of military trucks and armoured personnel carriers moving across the Euphrates.

In Cairo, Egypt, Iraqi Foreign Minister Naji Sabri said as he arrived for an Arab League meeting that developments yesterday — including coalition casualties and prisoners taken — demonstrated Iraqi resolve.

“What happened today showed that we’re not surrendering easily,” Sabri said. “It is proof we’re strong and it is not an easy invasion.”

“Clearly they are not a beaten force,” said General Richard Myers, chairman of the US Joint Chiefs of Staff. “This is going to get a lot harder.”

Separately, the US 3rd Infantry Division’s 2nd Brigade covered roughly 370 kilometres (230 miles) in 40 hours to take positions about 160 kilometres (100 miles) from the Iraqi capital yesterday.

The brigade raced day and night in more than 70 tanks and 60 Bradley fighting vehicles to reach the Shiite Muslim holy city of Najaf. At one point the soldiers ran into an hours-long firefight, killing 100 Iraqi militiamen who confronted the Americans with machine gun-mounted vehicles.

The brigade’s commander, Colonel David Perkins, likened the thrust to another surprise military move — that of a Carthaginian general who caught the Romans off guard by getting his army over the Alps in 218 BC.

“I’m using the analogy of Hannibal taking elephants over the Alps,” Perkins said. “But instead of the Alps, there are big wadis (gulches) out there and the elephants are the tanks.”

In Baghdad, explosions rocked the capitol early today and anti-aircraft fire could be heard in the smoke-filled capital. Earlier in the day, a series of air raid sirens and explosions were heard on the outskirts of the city.

Iraqi Defence Minister Lieutenant General Sultan Hashim Ahmed expressed confidence his troops can hold the capital.

“If they want to take Baghdad they will have to pay a heavy price,” he said.

Early yesterday, the Qatari-based satellite station al-Jazeera aired footage from Iraqi television of interviews with what the station identified as captured American prisoners, and also showed bodies in uniform in an Iraqi morgue that it said were Americans. The station said the prisoners were captured around Nasiriyah.

Abizaid said the airing of the al-Jazeera footage was a violation of the Geneva Conventions on prisoners of war. American television shows pictures of captured Iraqi soldiers.

Near the Gulf, Marines seized an Iraqi naval base yesterday morning at Az Zubayr. In the command centre, Marines found half-eaten bowls of rice and other still-warm food.

Near Basra in the south, Marines saw hundreds of Iraqi men — apparently soldiers who had taken off their uniforms — walking along a highway with bundles on their backs past burned-out Iraqi tanks.

Allied forces have captured Basra’s airport and a bridge. But commanders say they are in no rush to storm the city, hoping instead that Iraqi defenders decide to give up.

Coalition troops, after more than three days, were still trying to mop up resistance at the main Gulf port of Umm Qasr. They engaged in street-to-street battles against guerrillas, including paramilitary fighters of the Baath party.

Also yesterday, military officials confirmed the first “friendly fire” incident of the war against Iraq as a US Patriot missile battery shot down a British Royal Air Force Tornado GR4 near the Iraqi-Kuwait border. The two British fliers aboard were killed.