Al-Shabab also wants an exchange of prisoners
Somali Islamists have issued a set of demands for the release of a French security adviser they seized in July.
Al-Shabab say they will free the man if France stops its support for the weak UN-backed government and ends its anti-piracy patrols in Somali waters.
The group also wants African Union peacekeepers withdrawn from Mogadishu.
Another French security adviser captured on the same day managed to get free last month and is now in France. He denies reports a ransom was paid.
French Foreign Minister Bernard Kouchner said negotiations would not be carried out via the media but said he "hopes to secure the release of this last hostage".
He insisted that the Somali government of President Sheikh Sharif Sheikh Ahmed was legitimate and enjoyed the support of the people. It only controls small parts of the capital, Mogadishu.
Mr Kouchner said the two French agents had been training the Somali police force and presidential guard.
Al-Shabab, which is accused of having links to al-Qaeda, wants France to end all military and financial support for the government and to withdraw its security advisors from the country.
It also demanded an "exchange of prisoners" and the "freeing of mujahedeen prisoners in countries whose names will be announced later if Allah is willing".
The demands come three days after al-Shabab vowed to avenge a US raid in southern Somalia, which reportedly killed an al-Qaeda suspect.
Initial eyewitness reports suggested that the attack was carried out by French forces, although this was denied by Paris.
Anonymous US officials have confirmed that the helicopter raid was the work of US Special Forces.
France is one of several countries to have sent warships to the Indian Ocean following a spate of pirate attacks, mounted from Somalia.
The country has not had a functioning central government since 1991, leading to a complete breakdown of law and order both on land and in recent years in Somali waters.
President Ahmed, a moderate Islamist and former insurgent, was chosen in January after UN-brokered peace talks.
He has vowed to implement Sharia but al-Shabab accuses him of being a Western puppet.
Years of fighting and anarchy have left some three million people - half the population - needing food aid.