Unidentified gunmen have killed 18 people and injured six others working on a gas pipeline in Iraq, police sources have told Al Jazeera.

Gunmen in three vehicles used machine guns in the attack on Friday evening in the town of Imam Wais, 65km east of Baghdad.

Fifteen Iranian and three Iraqi workers were killed, while five Iranians and one Iraqi were wounded.

The workers were laying the foundation for a gas pipeline which is set to connect Iraq with neighbouring Iran.

Deadly day

Also on Friday, a series of car bombs and attacks in Iraq killed 18 people, and wounded 37 others, according to Al Jazeera police sources.

A car bomb exploded near a security checkpoint in Ramadi, west of Baghdad, killing six people, and injuring at least ten.

In a drive-by shooting in Baghdad's Ghazaliyah area, gunmen shot and killed two people, including a government employee, in their car.

Two separate car bombs also exploded near the capital: the first, near a fish market in Nahrawan, killed five people and wounding at least 14, while another exploded in Madain, just south of Baghdad, killing five people and injuring 13.

Violence in Iraq has reached a level this year not seen since 2008, when the country was just emerging from a brutal period of rampant sectarian killings. Nearly 950 people were killed in the month of November alone.

More people died in the first eight days of December 2013 than in the whole of last December, according to AFP news agency figures based on security and medical sources.

Prison break

Elsewhere in the capital on Friday, at least 22 Iraqi detainees facing terrorism charges escaped from a Baghdad prison, after killing at least one prison guard, authorities say.

Following a police manhunt, Iraqi security forces recaptured 13 prisoners and killed one, while another eight prisoners were still at large.

A police colonel said the detainees had seized weapons from guards before their escape from the al-Adela facility in the northern Baghdad area of Kadhimiyah.

The prisoners had lured a guard into their cell while his colleagues were sleeping, claiming an inmate was critically ill. They then stabbed the security guard to death, two senior security officials said.

Several guards were later detained and questioned over suspicions they had helped in the escape, a prison official said.

The jail break was the latest in a series of prison escapes in Iraq. More than 500 inmates escaped, including several al-Qaeda leaders, and dozens were killed, after an al-Qaeda linked group attacked two prisons last July.

The security breaches have cast doubts on the ability of the Iraqi authorities to secure the country amid the countrywide increase in violence.