BANGUI, April 14 (Reuters) - Central African Republic closed its border with Sudan on Friday in protest at what it called Khartoum's "aggression" following rebel attacks on its neighbour Chad, its foreign minister said.

"We are closing the border. We condemn the aggression against Chad," Central African Republic's Foreign Minister Jean Paul Ngoupande told Reuters, adding the country was stopping short of cutting diplomatic ties with Khartoum.

Chadian rebels attacked the capital N'Djamena early on Thursday in the boldest assault yet by fighters who have vowed to end Deby's nearly 16-year rule and block a May 3 presidential election in which he is standing for re-election.

Chad blames Sudan for supporting the rebels and President Idriss Deby said earlier he was breaking diplomatic ties with Khartoum. Some of the rebels entered Chad from their base in Sudan via Central African Republic, Ngoupande said.

Chad and Sudan accuse each other of backing rebels fighting governments in N'Djamena and Khartoum but agreed at a summit hosted by Libyan leader Muammar Gaddafi in Tripoli in February to stop insurgents setting up bases on their territories.