French judges have dropped an investigation into the missile attack which sparked Rwanda's 1994 genocide, a probe which has poisoned relations between the countries for two decades.

Juvenal Habyarimana, Rwanda's president at the time, was killed when his plane was shot down in April 1994, triggering 100 days of bloodshed that left an estimated 800,000 people dead.

Seven people close to current President Paul Kagame were charged in the French inquiry, including former defence minister James Kabarebe.

But they will not face trial after a legal source told AFP on Wednesday that French judges had dropped the probe on December 21 due to a lack of evidence.

Philippe Meilhac, a lawyer for Habyarimana's widow Agathe, told AFP that plaintiffs in the case would appeal the decision.