The offices of Niger's justice ministry in Niamey have been ravaged by a serious fire which broke out in the early hours of Tuesday morning.

The flames completely destroyed the first floor of the building, including the office of the minister and those of his closest aides, an AFP correspondent saw. Firemen were putting out the last of the blaze during the morning.

"The fire began shortly after 4:00 am (0300 GMT). All the offices in the building were burned, including that of the minister, reduced to ashes," said Ousmane Toudou, the ministry spokesman. "Everything burned, the archives and the cases under way, everything."

"If this fire is linked to the actions we are undertaking to clean up (the justice system), I think it was a waste of effort," Justice Minister Marou Amadou, who is also government spokesman, told journalists.

Amadou nevetheless declined to speculate on the cause of the fire, accidental or otherwise, while waiting for the outcome of a police inquiry that has got under way.

The government of President Mahamadou Issoufou, in power since 2011, has made much of its determination to fight corruption, notably among magistrates.

In August last year, the justice ministry set up a hotline number for callers wishing to report alleged cases of corruption.

At the end of December, the government asked that parliamentary immunity be lifted on eight lawmakers suspected of involvement in corruption and embezzlement.