French President Francois Hollande has ordered an increase in domestic security in the aftermath of military operations in Africa.
He was responding to the risk of Islamist attack after French forces intervened against militants in Mali.
A pilot was killed as France launched air strikes on a column of rebels, reports the BBC.
Two French soldiers were "sacrificed" in a raid to free a French hostage in Somalia on Saturday, Hollande said. The hostage was believed to have died.
France's anti-terrorism alert system known as "Vigipirate" is being raised to boost security at public buildings and transport networks.
The "struggle against terrorism" required all necessary precautions to be taken in France itself, the president said.
Earlier, one of the Islamist groups targeted by French military action in the west African state of Mali threatened reprisals against France.
An Ansar Dine spokesman told Reuters news agency there would be consequences for French citizens throughout the Muslim world.
The operations in Mali and Somalia were launched within hours of each other but were "totally unconnected", according to government officials.
French troops were deployed in Mali on Friday after the army lost control of a strategically important town to Islamists who were advancing south. The rebels took control of a huge swathe of northern Mali last April.
The central town of Konna has since been recaptured, the Malian government says.
Then, French commandos went into action in Somalia, swooping on the town of Bulo Marer in an attempt to free Denis Allex, who was kidnapped in July 2009.
A battle erupted with al-Shabab militants and, according to President Hollande, the operation failed "despite the sacrifice of two of our soldiers and without doubt the assassination of our hostage".
Earlier, the French defence minister said one of the soldiers had been killed, another was missing.
The Islamist group al-Shabab has said Allex was not in the area at the time of the raid and he is alive.
The president said he had taken the decision to launch the mission several days ago.
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