Two gunmen have been killed and a prosecutor critcally injured as a hostage stand-off in Turkey came to a violent end.

Istanbul's police chief said security forces stormed the courthouse where a far-left group had taken Mehmet Selim Kiraz hostage.

Selami Altinok said the prosecutor was undergoing surgery and fighting for his life.

"Because of gunshots heard from inside the prosecutor's office, our security forces carried out an operation," he said.

He added that police had negotiated with the gunmen for six hours before storming the building.

An unverified photograph that emerged earlier appeared to show Mr Kiraz with a gun to his head and a hand over his mouth - with his hands apparently bound with cable ties.

The photograph was posted online by the Revolutionary People's Liberation Party-Front (DHKP-C) - a banned far-left organisation.

The group, which is listed as a terror organisation by the US, the EU and Turkey, had threatened to kill Mr Kiraz at 1.36pm UK time if its demands were not met.

Mr Kiaz had been the prosecutor investigating the death of a teenager who was hit by a police gas canister fired during nationwide anti-government protests in 2013.

A website close to DHKP-C said militants from the banned organisation had given authorities three hours to meet five demands, including forcing policemen held responsible for the teenager's killing to confess.

Deputy Chief Prosecutor Orhan Kapici confirmed the incident was related to Mr Kiraz's investigation into the boy's death.