A sand storm brought hundreds of tanks and amphibious assault vehicles to a complete halt, slowing the US Marines Corps' drive on Baghdad.The storm blackened the skies with only occasional flickers of ghostly orange light and the sand filled the lungs of the service personnel traveling in the convoy, which also included seven-tonne trucks -- carrying vital supplies of fuel, food and ammunition.Earlier the fifth regiment of the Marines 1st Division had spent four hours struggling through the amber-lit desert landscape left by the sandstorm.

The 6,000-strong regiment had pushed north in a huge convoy since Sunday, but had opted for out-of-the way dirt roads winding through the ancient homeland of the Marsh Arabs and barren desert, as fighting flared in Nasiriyah, a strategic crossing point over the Euphrates river.

The regiment hit a highway once again Tuesday afternoon for the first time in more than 24 hours.Farther south, another dense sandstorm disrupted critical helicopter operations by the US 101st Airborne Division that is advancing on Baghdad, correspondents there said.

An Air Force meteorologist assigned to the 101st said visibility was less than a mile (half a kilometre) and that winds had reached speeds of 35 to 50 knots, conditions that were likely to prevail until Wednesday morning.

"After that it's supposed to clear -- clear skies and unrestricted visibility," Lieutenant Patrick Gioia told AFP."There will be slight winds of up to 10 knots but no gusts," he said, adding that the good weather should remain for the subsequent three to four days.

Gioia said it was "impossible" to fly under the current conditions."Once you get up to 300 to 500 feet (90-150 meters) you get blue sky and unrestricted visibility.

"But getting off the ground, getting up in the air and landing is the problem."The drive north from Kuwait by the 101st has been hampered by howling winds and swirling sand for the past two days.The 101st Airborne, with its fleet of Apache and Black Hawk attack helicopters, as well as Chinook troop transports, is moving on Baghdad along with other US army troops and marines further to the east.

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US soldier with the 101st Airborne brigade