A prominent Iranian human rights lawyer is in a serious condition after going on hunger strike in prison, a New York-based Iranian rights group says.

The International Campaign for Human Rights in Iran (ICHRI) called on the United Nations to intervene in the case of Nasrin Sotoudeh.

Ms Sotoudeh was arrested in September and is accused of acting against national security.

She has been refusing food since Sunday in protest against her treatment.

ICHRI spokesperson Hadi Ghaemi quoted Ms Sotoudeh's husband, Reza Khandan, as saying she has lost a lot of weight since being detained in Tehran's Evin prison, and that her skin has darkened.

Mr Ghaemi said she had only been able to see her two young children on Thursday for the first time since her arrest.

He urged UN High Commissioner for Human Rights Navanethem Pillay to take up the matter with the Iranian authorities immediately.

Mr Ghaemi also said the Iranian authorities had denied her contact with her lawyer and had held Ms Sotoudeh in solitary confinement.

Her trial is due to start on 15 November.

Ms Sotoudeh has represented Iranian opposition activists and politicians, as well as Iranian Nobel peace prize laureate Shirin Ebadi.