Sky Mali, the only commercial airline flying to Timbuktu in Mali’s interior, has cancelled flights there due to insecurity, it said on Monday, deepening the isolation of the northern city which has been under a month-long Islamist blockade.

Timbuktu, a UNESCO World Heritage site and ancient trading centre on the edge of the Sahara desert, has been suffering from a shortage of food and aid supplies since a local affiliate of al Qaeda cut off access by road and river in mid-August.

Two residents told Reuters that they heard shell fire near the city’s airport on Monday morning.

Sky Mali later issued a statement saying it had suspended all flights to and from Timbuktu until further notice, citing a security alert.

“We heard several shell shots at Timbuktu airport. Flights are cancelled,” said resident Mohamed Ag Hamaleck.

“Now Timbuktu is completely closed. The access roads are cut, the boats no longer come,” he said by phone.

The city has been surrounded by violence ever since French forces liberated it from militants in 2013 after an uprising. The Islamists later regrouped and have spread from northern Mali to neighbouring Burkina Faso and Niger.

The European Union said last week that the blockade had extended to more localities in the Timbuktu region, including Rharous, Niafounké, Goundam, Diré, Tonka, Ber and Léré.

“Civilians do not have access to essential products and basic social services,” the EU’s humanitarian branch ECHO said in a note.

Insecurity in Mali has intensified over the past year after the West African country’s military leaders kicked out French troops, asked United Nations’ peacekeepers to leave, and teamed up with Russian private military contractors Wagner Group.

An al Qaeda-linked group claimed responsibility on Friday for a suicide attack on a military base in northeastern Mali, a day after authorities blamed the group for carrying out a dual assault on another military camp and on a boat that killed more than 60 people.