Gunmen have launched an attack on the Radisson Blu Hotel in the centre of Mali's capital, Bamako.
"Apparently it's an attempt to take hostages. The police are there and are sealing off the area," a security source told Reuters news agency.
Automatic weapon fire could be heard from outside the 190-room hotel, AFP reports.
BBC Afrique's Abdourahmane Dia says the US-owned hotel is popular with expats working in Mali.
In August, suspected Islamist gunmen killed 13 people, including five UN workers, during a hostage siege at a hotel in the central Malian town of Sevare.
France, the former colonial power in Mali, intervened in the country after al-Qaeda-linked militants threatened to march on Bamako in January 2013 after taking control of the north of the country.
Mali attack: Live updates
The US embassy in Bamako has tweeted that it "is aware of an ongoing active shooter operation at the Radisson Hotel".
All US citizens were asked "to shelter in place" and "encouraged to contact their families".
"It's all happening on the seventh floor, jihadists are firing in the corridor," a security source told AFP.
Witnesses say the police have surrounded the hotel and are blocking roads leading into the neighbourhood.
The Rezidor Hotel Group, which owns the Radisson Blu, said it was aware of "the hostage-taking that is ongoing at the property today".
"As per our information two persons have locked in 140 guests and 30 employees," AFP quotes its statement as saying.
The UN force in Mali took over responsibility for security in the country from French troops in July 2013.
BREAKING NEWS: Jihadist gunmen launch grenade and shooting rampage in hotel and take 170 guests and staff hostage in Mali
Radison Blu hotel under attack by jihadists in Malian capital of Bamako
Gunmen are believed to have taken hostages inside the Radisson hotel
Explosions and machine gun fire were heard at the popular hotel in Mali
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