By Adama Diarra

KIDAL, Mali (Reuters) - Mali sent in troops on Sunday to retake Kidal from Tuareg separatists after six government workers and two civilians were killed, according to the United Nations, during an attack on the regional governor's office.

At least eight soldiers were also killed and around 30 civil servants captured by rebels during clashes that broke out while Prime Minister Moussa Mara was on a visit to the northern town.

A spokesman for the separatists denied that anyone had been killed inside the government building.

Gunfire had already broken out before Mara's arrival early on Saturday and he was forced to take shelter in an army base.

"In light of this declaration of war, the Republic of Mali is henceforth at war," Mara told a Reuters reporter inside the base overnight.

He told a news conference on Sunday after he moved to Gao, another city in the north, that the government had already sent troops, including special forces, to retake Kidal.

"Reinforcements are on the way to Kidal. The objective is to totally retake Kidal," a senior military source also told Reuters, asking not to be named.

Mara was visiting the town, a stronghold of Tuareg separatists, for the first time since his appointment last month as part of efforts to revive long-delayed talks with northern armed groups.

Mali was thrown into turmoil in 2012 when al Qaeda-linked Islamists took advantage of a Tuareg-led rebellion and seized control of the country's north before a French-led military operation, known as Serval, drove them back last year.

The government and a grouping of armed groups including the Tuareg National Movement for the Liberation of Azawad (MNLA), which broke with the Islamists ahead of the French offensive, signed an agreement to hold talks over autonomy last year.

But the clashes, the most serious pitting the government against Tuareg fighters since the French intervention, now threaten to sink efforts to find a peaceful solution to the long cycle of rebellions in the West African nation's desert north.

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The United States condemned the violence, saying it undermined the country's fragile peace.

"We call for the immediate release of all hostages, and urge all parties to refrain from violence and from any acts that place civilians at risk," State Department spokeswoman Jen Psaki said in a statement. "The way to resolve these issues is through an inclusive and credible negotiation process, not through violence and intimidation."

The flare-up in a trouble spot many had hoped had been brought under control comes as West African nations and their international partners are redoubling efforts elsewhere to contain Islamist groups such as Boko Haram in Nigeria.

France, in particular, is seeking to redeploy part of its force in Mali to tackle the regional threat.

MINUSMA, a nearly 13,000-strong U.N. peacekeeping mission, is rolling out, but is not yet at full strength.

'BARBARIC CRIME'

Mara criticized both the French and U.N. forces for allowing the attack to take place.

"The very least we'd expected from MINUSMA and Serval was that they'd ensure the governor's office wasn't attacked," he said.

MINUSMA said on Sunday that 21 U.N. police officers were injured in the clashes while providing security for the prime minister's visit to Kidal. Two suffered serious gunshot wounds.

"This barbaric crime is totally unacceptable and those responsible must answer for their actions," Albert Koenders, the head of MINUSMA, said of the killings in the governor's office. "An inquiry must be carried out quickly in order to verify the facts and bring the responsible parties to justice."

A spokesman for the MNLA, which claimed control of Kidal on Sunday, had earlier said the rebels were preparing to hand over the government workers they were holding.

"There were no murders," Attaye Ag Mohamed told Reuters by telephone from the town. "Those killed at the governor's office were killed in the exchange of gunfire or mortar explosions."

He said the MNLA was also holding 15 soldiers it considered to be prisoners of war.

Malian forces suffered 25 wounded in addition to the eight dead, according to the Defense Ministry, while 28 attackers were killed and 62 wounded.

A Malian military source said Saturday's gun battle erupted after MNLA fighters in two trucks attacked an army checkpoint in front of the governor's office.

The MNLA's Ag Mohamed rejected the government's version of Saturday's events and said the army attacked first, opening fire on the group's barracks following pro-independence protests in the town.

He said the rebels had killed 19 government soldiers and suffered no losses of their own.

"The situation is calm right now. We're in position. We're not scared of the Malian army. We're ready," Ag Mohamed said.

(Additional reporting by Tiemoko Diallo and Joe Bavier, and Peter Cooney in Washington; Writing by Joe Bavier; Editing by Alison Williams and Eric Walsh)