Amnesty International accused Israel Friday of subjecting nuclear whistle-blower Mordechai Vanunu to "inhuman" treatment by sending him back to jail.

Vanunu, a former nuclear technician who served 18 years for disclosing atomic secrets to a British newspaper, was jailed for three months last month for breaking the terms of his release and meeting a foreigner.

"Mordechai Vanunu should not be in prison at all, let alone be held in solitary confinement in a unit intended for violent criminals," said Malcolm Smart, the London-based rights group's Middle East programme director.

"He suffered immensely when he was held in solitary confinement for 11 years after his imprisonment in 1986 and to return him to such conditions now is nothing less than cruel, inhuman or degrading," he added.

An Israeli court had in December sentenced Vanunu to serve three months community service or three months in prison, for violating the terms of his release from prison in 2004.

The Israeli had opted for community service but asked Israel's Supreme Court if he could perform it in Arab east Jerusalem, as opposed to mainly Jewish west Jerusalem where he feared he would be "harassed by the Israeli population."

The court rejected his request and ordered him to serve three months behind bars -- although Amnesty's assertion that he has been held in solitary confinement has not been confirmed.

"Mordechai Vanunu is a prisoner of conscience," said Smart.

Prison authorities "might claim that he has been put in isolation to protect him from the risk of attack by other inmates, but if the Israeli government is really concerned for his safety it should release him without delay.

"His re-imprisonment is both harsh and unjustified," he added.

Vanunu was jailed in 1986 for disclosing the inner workings of Israel's Dimona nuclear plant to Britain's Sunday Times newspaper.

Since his release in 2004, he has been detained several times for violating the terms of his release that ban him from travel or contact with foreigners.

Israel is widely believed to be the only nuclear-armed power in the Middle East, with around 200 warheads, but it has a policy of neither confirming nor denying that.

The Jewish state has refused to sign the nuclear Non-Proliferation Treaty or to allow international surveillance of Dimona in the southern Negev desert.