Islamist militants staged twin suicide car bombings on an army base and a French-run uranium mine in Niger on Thursday, killing at least 20 people and taking several trainee officers hostage in the impoverished west African state.
An Islamist group claimed the unprecedented attacks as revenge against the country's involvement in France's offensive against militants in neighbouring Mali.
They come just four months after Al-Qaeda linked militants seized a desert gas plant in neighbouring Algeria in a siege that left 38 hostages dead, also in retaliation against the intervention in Mali.
The Movement for Oneness and Jihad in West Africa (MUJAO), one of the Islamist groups which seized control of northern Mali last year before being driven out by French-led troops, claimed the near simultaneous bombings at the Agadez army base and the French majority-owned uranium mine in Arlit.
"Thanks to Allah, we have carried out two operations against the enemies of Islam in Niger," MUJAO spokesman Abu Walid Sahraoui told AFP.
"We attacked France and Niger for its cooperation with France in the war against sharia (Islamic law)."
French President Francois Hollande vowed to help Niger "destroy" the militants and said France would back "all the efforts of Niger to stop the hostage situation" at the army base.
"We will not intervene in Niger as we did in Mali, but we have the same willingness to cooperate to fight against terrorism," he said.
The first car bomb went off at dawn at the army base in Agadez, the largest city in mostly desert northern Niger.
Eighteen soldiers and a civilian were killed along with four attackers at the army base, Interior Minister Abdou Labo said.
"A fifth bomber has locked himself up in an office with several trainee officers as hostages (at Agadez)," Labo said. "We are taking action to arrest the bomber and free the hostages."
About 30 minutes after the first attack, a suicide bomber blew up an explosives-laden four-by-four at the Somair uranium mine and processing facility as employees reported for work at the site, which is majority-owned by France's Areva.
Areva said one person was killed at the mine located some 250 kilometres (150 miles) north of Agadez, but did not identify the victim. It added that 14 others were wounded.
Labo said however that around 50 people were wounded at the mine, adding that almost all of the victims were security agents.
Defence Minister Mahamadou Karidjo described the attackers as "redskins", in a reference to members of the country's Tuareg and Arab groups.
Agadez resident Barka Sofa said he heard a strong explosion outside the army base followed by heavy weapons fire, while a local journalist reported heavy damage inside the camp.
"All the streets of Agadez are blocked. The army is sweeping the city," Sofa added.
At the mine, an employee told AFP that "a man in military uniform driving a four-by-four packed with explosives mixed in with the Somair workers and blew up his vehicle in front of the power station at the uranium treatment facility."
"Company managers told us the suicide bomber was killed in the explosion," he added, saying the blast had caused damage but had not stopped work at the site.
Somair is 63.6-percent-owned by Areva and 36.4-percent-owned by SOPAMIN, the agency that manages Niger's state mining interests.
Areva, the world's second-largest uranium producer, extracts more than a third of its uranium in the impoverished west African country and has operated in Niger for more than 40 years.
The attacks come some four months after the seige in neighbouring Algeria that left 38 hostages dead, including 37 foreigners.
Areva condemned the blast as a "terrorist attack" on its website and said Nigerien authorities had stepped up security measures at its facilities.
Niger is part of the African-led Support Mission to Mali (AFISMA), a regional military mission launched to help reclaim northern Mali from Al-Qaeda in the Islamic Magreb (AQIM) and two allied Islamist groups that seized the vast desert territory in the chaotic aftermath of a March 2012 military coup.
French troops have so far led the operation against the Islamists, which was launched in January and has pushed the radicals from the territory they had brutally ruled.
Islamist groups have carried out several kidnappings in Niger in recent years, especially in the north.
Seven employees of Areva and one of its subcontractors were abducted in September 2010 by AQIM. Four Frenchmen are still being held by their kidnappers.