Seven employees of French construction company Vinci and nuclear energy firm Areva have been kidnapped in northern Niger.
The group, which included five French citizens, was seized overnight in the town of Arlit, in the Sahara desert.
Areva operates a uranium mine near Arlit. A subsidiary of Vinci, Sogea-Satom, is employed as a subcontractor.
President Nicolas Sarkozy said he has asked the Niger authorities "to do all they can to free them".
A French foreign ministry spokesman confirmed the kidnappings and said its officials were "totally mobilised".
When asked who he thought was behind the abduction, Bernard Valero told the Reuters news agency: "The security threat is so vague, so imprecise. Political terrorism is always very difficult to establish.
"For the moment we are in the first few hours. We've just had the information. There has not been any claim, so it's something that we can't really explore for the moment, but we will consider every possibility."
A spokesman for Niger's government told the AFP news agency that the other two hostages were from Togo and Madagascar and that they had been taken by an "armed group" of up to 30 men.
Laouali Dan Dahdit said the kidnappers "spoke mostly Arabic and Tamashek", a Berber language used by the desert region's semi-nomadic Tuareg people.
They were last seen heading towards Inabangaret, an important well and stopping point near the border with Algeria and Mali, he said.
"A certain number of measures have been taken by the military as well as the police so that the victims and the kidnappers can be found quickly."
Speaking at an EU summit in Brussels, Mr Sarkozy said the kidnappers had not yet been identified and there was "no certainty" on the matter.
"I asked the Niger authorities to do everything in their power to attempt to find these hostages and to free them. A local investigation is under way," he said.
Mr Sarkozy repeated earlier warnings made by French Foreign Minister Bernard Kouchner to French nationals in the Sahel region "that the situation is extremely dangerous, and that they should stay there only in very specific circumstances, and that they must have protection".
"The situation we are now experiencing shows that we must redouble vigilance," he said.
A Vinci spokeswoman said its employees had been working on an earth-moving project under Areva's supervision. Security had now been stepped up at the mine, she added.
A local resident told Reuters the attack took place in the middle of Arlit.
"They went to their houses and grabbed them. They knew exactly where they were - it is very worrying," he said.
In recent years, rebel groups in the region have been fighting for a greater share of Niger's uranium wealth. The country is the world's sixth biggest producer of uranium, and the radioactive heavy metal is its main export.
Areva, the world's largest nuclear manufacturer, gets much of its uranium from Niger.
In 2008, Areva announced the release of four of its employees - all French nationals - who had been kidnapped by the Movement for Justice, which opposes the mining of ancestral lands.
Al-Qaeda in the Islamic Maghreb (AQIM), an offshoot of the Islamist militant group, is also active in the region and has kidnapped French and other European nationals in the past.
In July, the group announced that it had executed a 78-year-old retired French engineer it was holding hostage in Mali, after a raid by the French and Mauritanian armed forces failed to free him.
The following month, the Spanish government is believed to have paid millions of euros to free two of its nationals seized by AQIM in Mauritania.