French intelligence services are searching for a female would-be suicide bomber who they believe is planning to attack the Paris transport system. The country is on full alert against a specific terrorist threat that has been confirmed by two separate sources, according to French radio.

The rector of the Grand Mosque of Paris, Dalil Boubakeur, considered a moderate Muslim, has also been given three armed guards after a spokesman said he was under a "real threat".

France's interior minister, Brice Hortefeux, refused to give details of the threat but said "our vigilance is reinforced today". The anti-terrorist court in Paris has opened a preliminary inquiry to "establish the credibility" of the reports of an imminent terrorist attack.

According to a ministry of interior spokesman, the alleged woman bomber was "not necessarily the most worrying" intelligence information. He did not give further details. "All the lights are flashing red," said another intelligence source.

Last week, Hortefeux admitted the threat was at a "high level" when he visited the Eiffel tower after it was evacuated following a bomb alert.

The Saint-Michel metro station near Notre Dame cathedral was also briefly evacuated over a similar threat.

France's terrorism level has been on what is described as "reinforced red" since 5am last Thursday. This followed intelligence from a North African country, said to be either Morocco or Algeria, that an Algerian was planning to carry out an attack on a Paris train, bus or metro that day. French intelligence had been given similar information from another source.

RTL radio said: "The secret services spent the whole day searching for the suspected woman terrorist … in vain. The extremely credible threat is still being taken seriously today."

French intelligence agencies say the threat is linked to the al-Qaida of Islamic Maghreb organisation thought to be responsible for the kidnapping of seven nuclear workers – five of them French, one Togolese and one Malagasy – in Niger last week. There has been no claim of responsibility for the attack on the foreign workers – six men who were employed by the French nuclear company Areva and one woman – in the northern town of Arlit, 500 miles north-east of the capital Niamey, but French officials suspect al-Qaida's north African branch.

French anti-terror troops are co-ordinating an aerial search for the hostages who were abducted in the Sahara desert. Around 80 French anti-terror specialists set up base in a hotel in the capital, Niamey, at the weekend following an invitation from Niger's government to pursue the kidnappers on its soil.

Long-range reconnaissance planes equipped with infrared sensors are searching the desert in the north, where the hostages were seized from their homes on Thursday, as well as the rugged fringes of neighbouring Mali, where it is thought they may be being held. "Their mission is to help Niger's military find the seven kidnapped people," said Niger government spokesman Mahamane Laouali Dan Dah.

French terrorism experts say al-Qaida's north African branch has no effective logistical network in France but could carry out an attack "either by using individuals already present in France or by sending a group specifically dedicated to carrying out one".

Bernard Squarcini, head of France's counter-terrorism services, said the threat of an attack in France "had never been higher" in an interview with Le Journal du Dimanche in early September.

France also fears the threat is heightened by the return of Islamist extremists who have been waging jihad in Afghanistan and Iraq.

There is also anger over the planned ban on wearing the burqa in public places, the presence of French troops in Afghanistan, and a French commando attack on an al-Qaida base in Mali in July, which led to the death of seven members of the organisation's north African branch.