Gunmen have launched an attack on the Radisson Blu Hotel in the centre of Mali's capital, Bamako.
The hotel's owners said that two people had locked in 140 guests and 30 employees, the AFP news agency reports its statement as saying.
Police have surrounded the hotel, which the gunmen entered shooting, and shouting "God is great!" in Arabic, a security source told Reuters.
A BBC reporter 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 in January 2013 when al-Qaeda-linked militants threatened to march on Bamako 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.
Some reports say about 10 gunmen in total are believed to be involved in the incident.
The Rezidor Hotel Group, which owns the Radisson Blu, said it was aware of "the hostage-taking that is ongoing at the property today".
The UN force in Mali took over responsibility for security in the country from French and African troops in July 2013, after the main towns in the north had been recaptured from the Islamist militants.
Militancy in Mali:
October 2011: Ethnic Tuaregs launch rebellion after returning with arms from Libya
March 2012: Army coup over government's handling of rebellion, a month later Tuareg and al-Qaeda-linked fighters seize control of north
June 2012: Islamist groups capture Timbuktu, Kidal and Gao from Tuaregs, start to destroy Muslim shrines and manuscripts and impose Sharia
January 2013: Islamist fighters capture a central town, raising fears they could reach Bamako. Mali requests French help
July 2013: UN force, now totalling about 12,000, takes over responsibility for securing the north after Islamists routed from towns
July 2014: France launches an operation in the Sahel to stem jihadist groups Attacks continue in northern desert area, blamed on Tuareg and Islamist groups
2015: Terror attacks in the capital, Bamako, and central Mali