Deadly clashes have broken out in northern Mali between the army and rebels, who seized 28 people as the prime minister visited the area.
At least 36 people reportedly died in Kidal as Tuareg rebels fought troops on Saturday before and during PM Moussa Mara's visit.
When he left the governor's office in Kidal, the building was seized by the rebels, an eyewitness told the BBC.
In 2012 a Tuareg rebellion in northern Mali triggered a military coup.
Civilian rule was re-established in 2013, but Islamist and separatist forces remain active in some areas.
"Eight members of the armed forces were killed and 25 were wounded, while 28 of the attackers were killed," the defence ministry said.
Mr Mara said that the government was now "at war" with the separatists. President Ibrahim Boubakar Keita is expected to give a national address on Monday.
Malian Defence Minister Soumeylou Boubeye Maiga said on Sunday that reinforcements would be sent to Kidal.
"We will double our troops on the ground if necessary," AFP news agency quoted him as saying.
Mr Mara spent Saturday night at the Malian army camp in Kidal, flying to Gao early on Sunday morning.
The rebels are from the National Movement for the Liberation of Azawad (MNLA).
MNLA spokesman Moussa Ag Assarid told the BBC those seized- four civil servants and 24 Malian soldiers - were being held because the Malian government had failed to live up to its commitment to move towards peace talks.
Initially 32 people had been seized but the MNLA spokesman said four who were injured had been released to the Red Cross.
The United Nations is attempting to negotiate a truce between the Malian army and Tuareg rebels the BBC's Alex Duval Smith reports from the capital, Bamako.
The civil servants were posted in the town by Mali's government as a mark of sovereignty, our correspondent adds.
Further shooting was heard on Sunday and the governor's office was still under rebel control, but the intensity of fighting was unclear.