steel around Marjah, a town of 80,000 in Helmand province just 20 kilometres (12 miles) from the provincial capital of Lashkar Gah.
They are expected to launch an assault, called Operation Mushtarak ("Together") against militants who with drug traffickers have for years controlled the area, a source of most of the world's opium.
Up to 400 families have left Marjah, seeking shelter in Lashkar Gah and elsewhere, officials said, while those left were being told to stay put.
"Leaflets are being dropped over Marjah, containing the message 'Don't shelter the Taliban in your homes, don't allow the Taliban in your areas, the troops are coming to help you. We will bring peace. Live in peace and comfort'," said provincial spokesman Daud Ahmadi.
The same message is being broadcast on local radio, he said.
"Local people are also being encouraged to inform Afghan troops about Taliban IEDs," he said, referring to improvised explosive devises, the insurgents' main weapon in their fight to topple the Western-backed government.
Troops massed around Marjah have spent recent days sweeping roads and fields for IEDs, which cause huge losses among both troops and civilians.
While the offensive is expected to begin soon, troops have not yet entered the town, Ahmadi said.
Marines holding a strategic junction outside Marjah are using loud speakers to tell residents to stay indoors, to not shelter militants and that they and Afghan security forces have come to rid the area of "terrorists," an AFP photographer on the scene said.
The Taliban have vowed to stay and fight, with spokesman Yousuf Ahamdi, telling AFP: "We're fully prepared to fight them if they enter the town."
"We're firing rockets and other heavy weapons on them," he said.
"They have not reacted so far. We have laid mines. We have experience from previous operations, we'll be fighting them," he said by telephone from an undisclosed location.