KIGALI (Reuters) - Lawyers for former Congolese warlord Laurent Nkunda said on Tuesday they had filed a motion in Rwanda's Supreme Court seeking his release from house arrest.

Nkunda has been held in Kigali since January after he fled the neighbouring Democratic Republic of Congo, where he is accused of committing various crimes during his five-year rebellion in the mineral-rich east.

"My client has the right to know why he has been arrested, why he is detained and how long this illegal imprisonment will last," lawyer Stephane Bourgon said in a statement, adding there was no valid arrest warrant or charges against Nkunda.

Rwanda says his detention, seen as part of a deal to mend relations between former foes Rwanda and Congo, is fundamental to regional peace. Both countries began working together on energy projects on Lake Kivu earlier this year.

At Congo's invitation, Rwanda sent 3,500 troops across the border in January to help fight Hutu militia, regarded as the root cause of a 15-year-old regional conflict in which about 5.4 million people have been killed.

The government in Kigali says several legal and political issues needed to be resolved before Nkunda can be extradited. In late 2008 the United Nations accused Rwanda and Congo of supporting opposing rebel groups in Congo's east.