MOSUL, Iraq (Reuters) - Unidentified gunmen killed two Iraqi television journalists as they were filming in the northern city of Mosul on Saturday, security sources said.

The journalists worked for Iraqi television channel al-Sharqiya News, which is often critical of the government and
is popular among the Sunni minority. It was not clear who was behind the killings.

"The gunmen exploited this area as it is one of the city's busiest and shot the two journalists dead," said a security source who declined to be named.

"They shot them in the chest and head, killing them instantly".

Iraq is considered one of the most dangerous countries for journalists. According to the Baghdad-based Journalism Freedoms Observatory, 261 journalists have been killed and 46 kidnapped since 2003, the year of the United States-led invasion of Iraq.

Mosul, capital of the predominantly Sunni province of Nineveh, is a stronghold for Islamist and other insurgents who have been regaining momentum this year in a campaign to undermine Iraq's Shi'ite-led government.

A journalist from Mosul said insurgents in the city changed their tactics and targets from time to time, and may now have set their sights on journalists, after previous spates of attacks against traffic police and mayors.

"I will leave the city of Mosul and live in the outskirts until things calm down," said the journalist, on condition of anonymity.

The Journalists' Syndicate denounced the killings as a"criminal act", demanding the authorities track down the
perpetrators and do more to protect journalists and the media.

Nineveh Governor Atheel al-Nujaifi condemned the killings:"It aims to muzzle the voice of people, the voice of
righteousness".

The United Nations Mission in Iraq said nearly 900 civilians were killed across Iraq in September, raising the death toll so far this year to well above the total for 2013.