N'DJAMENA (Reuters) - Chad said on Monday it was closing its border with Sudan after Khartoum broke off diplomatic ties on Sunday following a rebel attack on its capital which it said was supported by N'Djamena.

Chadian President Idriss Deby's government denies involvement in Saturday's attack by Darfur rebels and has blamed Sudan for backing an assault on Chad's capital in February in which hundreds of people were killed.

"The government has decided the hermetic closure of the frontier in order to avoid any infiltration or suspect traffic," said a statement issued after an extraordinary cabinet meeting.

Chad also banned Sudanese airlines from flying to Chad and froze the assets of a Sudanese agricultural bank.

N'Djamena had initially expressed surprise at Sudan's "hasty decision" to break off diplomatic relations and said it hoped ties would be reestablished.

Deby and Bashir signed a non-aggression pact in March, pledging not to let their territory be used by rebels hostile to each other. Each has accused the other of breaking the deal.