Islamist fighters in northern Mali have planted anti-personnel mines around the town of Gao which they took control of last week, residents have said.
The move is intended to stop a potential attack by Tuareg rebels with whom they have fallen out.
Tuareg rebels and Islamist groups initially fought together to take over northern Mali in April after a coup.
Meanwhile, France says it is determined to stop the establishment in Mali of "international terror bases".
French Prime Minister Jean-Marc Ayrault said he wanted to prevent such groups as al-Qaeda in the Islamic Maghreb (AQIM) threatening the peace and security of the whole region, as well as France.
His comments came after al-Qaeda-linked militants from the Ansar Dine group destroyed ancient Muslim shrines in the northern city of Timbuktu, saying they contravened their interpretation of strict Islamic law.
The new chief prosecutor of the International Criminal Court, Fatou Bensouda, condemned the destruction as a "war crime".
Residents in Gao say fighters from another Islamist militant group, Mujao, have imposed a climate of fear by planting mines around town, which is east of Timbuktu.
The militants warned on local radio that people should not wander outside the main roads.
"They said it was to prevent the Tuareg rebels from trying to attack the town," resident Allouseini Mohamed told the BBC.
"It's a big problem because most people here are herders and cattle-breeders and they can't go out to their fields anymore. It's like a prison. People are scared."
The BBC's West Africa correspondent Thomas Fessy says the fear is such that markets and small shops have closed.
Tuareg fighters, who are seeking a secular independent north, have condemned the recent attacks on the ancient sites in Timbuktu and have vowed to fight back.
But our reporter says it is not known if they possess the ability to face the firepower of Islamist groups linked to al-Qaeda.
Tuareg rebels and Islamist militants capitalised on the chaos following an army coup in March and took control of the north of the country, but their uneasy alliance has collapsed.
Ansar Dine and Mujao now control all three of the region's main cities - Timbuktu, Gao and Kidal.
More than 300,000 people have fled northern Mali since the rebels took over the territory.