Gunmen shot dead two Iraqi journalists on Tuesday in Diyala, a province where Baghdad declared victory a year ago but which is still plagued by chronic violence.

Iraq is one of the most dangerous countries in the world for journalists, especially those from the country, who are far more exposed to attacks than their foreign counterparts.

The murders came as a suicide bomber killed four police and wounded a top intelligence officer elsewhere in the province, a day after other bombings claimed 20 lives.

"Armed militias assassinated correspondent Saif Tallal and his cameraman Hassan al-Anbaki near Baquba," the capital of Diyala province, a Sharqiya news presenter said on the air.

The journalists were killed while returning to Baquba from a reporting trip with Staff Lieutenant General Mizher al-Azzawi, the head of security command responsible for the province, the channel said.

Minas al-Suhail, a colleague from the channel, told AFP that the two journalists were driving some distance behind the commander's convoy on their way back from covering violence in the Muqdadiyah area.

Masked militiamen in three SUVs stopped their vehicle in the village of Abu Saida, took the journalists out and shot them dead with Kalashnikov assault rifles, Suhail said.

Shiite militia groups, some of which have been repeatedly accused of serious abuses, wield huge influence in the eastern province of Diyala.

Sharqiya is a Sunni-owned TV channel viewed as sympathetic to the country's Sunni Arab minority.

The murders took place within sight of a police checkpoint, but the police did not intervene, Suhail said.

According to Reporters Without Borders (RSF), 11 journalists were killed in Iraq during 2015, the most of any country.

"We deplore the murder of these two Iraqi journalists," Alexandra El Khazen, the head of RSF's Middle East bureau, said in a statement.

"This shocking double murder must not go unpunished. Iraq is a minefield for journalists. We urge the authorities to conduct an independent investigation in order to solve this crime and bring those responsible to justice," El Khazen said.

- Senior intelligence officer wounded -

The Committee to Protect Journalists put the death toll for those killed because of their work last year in Iraq at five, placing it in a tie for the fourth-deadliest country for the media.

CPJ also lists Iraq as the deadliest single country for journalists from 1992 to 2015, with 171 killed because of their work, almost double the second, which is Syria.

Muqdadiyah, the Diyala area from which the journalists were returning, was hit by deadly bombings and other unrest the night before.

Twin blasts killed 20 people at a cafe, and attackers subsequently blew up multiple Sunni mosques and burned houses and shops, officers said.

The United Nations issued a statement condemning the mosque bombings.

"Once again, places of worship are being attacked. The perpetrators want to incite sectarian violence, in a desperate attempt to take the country back into the dark days of sectarian strife," UN Iraq representative Jan Kubis said.

A suicide bomber also detonated an explosives-rigged vehicle near the convoy of the head of police intelligence in Diyala at a checkpoint in the province on Tuesday.

The blast in the Jdaidat al-Shatt area, south of Diyala capital Baquba, killed four policemen including a first lieutenant and wounded Colonel Qassem al-Anbaki and nine others, two officers said.

A doctor at Baquba General Hospital confirmed the toll.

There was no immediate claim of responsibility for the attack, but suicide bombings are a tactic frequently used by the Islamic State jihadist group.

Iraq declared victory over IS in Diyala early last year, but the persistent strife in the province paints a grim picture of the country's future even after defeating the jihadists.

Diyala remains a hotbed of violence by both the jihadists and powerful Shiite militia forces that have played a major role in the fight against IS.

IS overran large areas north and west of Baghdad in June 2014, but Iraqi forces backed by US-led air strikes have since dealt the jihadists significant defeats.