Voicing deep concern over mentally unsound Pakistani nationals languishing in Indian jails even after completion of their sentences, the Supreme Court on Tuesday asked the government why they should not be repatriated, saying such detention "pains us".

A bench headed by Justice R M Lodha said such matters should be taken up on priority basis and at the highest level when the top authorties of the two nations meet.

The bench was referring to 21 prisoners, 16 of whom are mentally unsound and five are deaf and dumb and are languishing in jail despite serving out their sentences.

"Should not such matters be taken up at the highest level when the heads of the state meet ?," the bench asked while indirectly referring to recent visit of Pakistani President Asif Ali Zardari to India.The court asked the Centre to find out in three weeks what can be done for sending back these prisoners to their country and posted the matter for further hearing on May 2.

"There is no doubt that the best of facilities are being provided to such prisoners in detention centre but the problem is why they are not being repatriated. What is the impediment? Such detention pains us," the bench said.

"Top most priority has to be given to these cases. They are mentally unsound and deaf and dumb. They have served out their sentences. They have been kept in jail because of some problem but that cannot be an indefinite exercise," the bench said.

The Centre contended that these people cannot be sent back without their identification being proved.

The bench then said, "How would you be able to do so even after six months or one year. Problem would continue. You must tell us what should be done."