Around 30 hostages kidnapped by Tuareg rebels in a deadly siege at local government offices in northern Mali over the weekend have been released, the United Nations has said.

"We have just taken back around 30 hostages. They are in not bad condition. Two are very tired," an official from MINUSMA, the United Nations peacekeeping force in Mali, told AFP news agency on Monday.

There were no immediate details on any negotiations behind the release, or confirmation that every hostage kidnapped in the northern rebel stronghold of Kidal on Saturday had been freed.

Deputy Secretary-General Jan Eliasson phoned Mali's President Ibrahim Boubacar Keita on Monday to express the UN's extreme concern and condemnation of the violence in the northern city of Kidal, UN  spokesman Stephane Dujarric said earlier in the day.

Separatist Tuareg rebels launched an assault on Kidal on Saturday, storming government buildings, killing soldiers and civilians, and taking hostages, officials said. The Malian army was preparing to launch an assault on Kidal.

A Tuareg invasion of northern Mali in 2012 was hijacked by al-Qaeda-linked fighters who were ousted by a French-led military invasion.

Source: Agencies