Rwanda's President Paul Kagame has warned of a "showdown" with France after a French investigation into the events leading to Rwanda's genocide was reopened.
Then-Rwandan president Juvenal Habyarimana was killed when his plane was shot down in 1994.
His assassination triggered nationwide violence, leading to 800,000 deaths.
Investigators want to hear evidence from a former general who claims Mr Kagame was involved.
President Kagame rejected the allegations, and criticised French authorities for re-opening the investigation last week.
"The judicial system of Rwanda is not subordinate to France or France's interests," he said.
France, in turn, is accused by Mr Kagame of taking part in the massacres - a charge denied by Paris.
"If starting all over again is a showdown we will have a showdown, there is no problem about that," Mr Kagame said.
Former Rwandan army chief Faustin Kayumba Nyamwasa, who lives in exile in South Africa, says that Mr Kagame was responsible for the attack on the aircraft.
French authorities have now asked for South Africa's cooperation in formally questioning Mr Nyamwasa.
Rwanda and France have an ongoing dispute about the circumstances surrounding the 1994 plane crash, in which French crew members also died.
Rwanda's genocide, the victims of which largely came from the minority Tutsi ethnic group, took place between April and June 1994, leaving 800,000 dead within 100 days.
On 6 Apri, 1994, President Juvenal Habyarimana's plane - which was also carrying the President of Burundi - was shot down by an unknown assailant. Mr Kagame maintains it was the work of Hutu extremists, who wanted to provide a pretext for violence against the Tutsi community.
A Rwandan inquiry backed up the president's assertions, but subsequent French investigations have failed to reach any conclusions.
In 2006, Rwanda cut ties with France for three years after a judge called for Mr Kagame to stand trial.
Relations between the two nations have remained tense since then.