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By Stephanie Hancock

N'DJAMENA, Feb 14 (Reuters) - Chadian President Idriss Deby on Thursday decreed a state of emergency across the central African state to tighten security after a rebel attack on the capital N'Djamena this month.

In an address on state television and radio, Deby said his government would take on exceptional powers for 15 days from Friday, as set forth under the constitution of the former French colony.

"These are exceptional measures which I must take to assure the normal functioning of the state," the president said.

A lighting assault by a convoy of 3,000 rebel fighters reached the outskirts of Deby's presidential palace on Feb. 2 before being turned back in two days of confused street fighting in which more than 160 people were killed.

Article 87 of Chad's constitution allows the president to adopt exceptional powers to preserve the territorial integrity of the nation. After an initial 15-day period, they require the approval of the National Assembly to be extended.

Deby said he would appoint presidential missions to monitor the implementation of the measures, including travel restrictions, a curfew and media censorship.

Deby, a former French-trained helicopter pilot who has held power for 18 years, last imposed a state of emergency in 2006, in the wake of the previous attack on N'Djamena by the fractious rebel alliance based in Sudan's western Darfur region.

A week ago, the government announced a dusk-to-dawn curfew in the capital and swathes of eastern and central Chad.

Since their failed assault on N'Djamena, the rebels have withdrawn to the desolate east, blaming France's military support for Deby for preventing his overthrow.

The rebels say the French intervention has made an EU peacekeeping mission, which restarted its deployment to eastern Chad on Tuesday, into a military target.

French troops make up the bulk of the 3,700-strong contingent tasked with protecting some 500,000 refugees from Darfur, eastern Chad and Central African Republic.

With its involvement in Chad under scrutiny, France's army admitted on Thursday that it had transported Libyan munitions to Chad to supply the army's defence of the capital, but it denied that its soldiers fought to keep Deby in power.

On Tuesday, France demanded the immediate clarification of the whereabouts of Chadian opposition leaders detained by soldiers during the dying hours of fighting in N'Djamena.

Chad's Interior Minister Mahamat Ahmat Bachir said they were abducted from rebel-held parts of the city and the government had opened a judicial enquiry. (Writing by Daniel Flynn; editing by Sami Aboudi)

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