A rocket barrage killed at least 26 Iranian exiles at a camp near Baghdad, UN chief Ban Ki-moon said Friday after the deadliest attack against the opposition group in years.

The United States and United Nations condemned the attack on the People's Mujahedeen Organisation of Iran (PMOI), which demanded that its members be protected.

The attack may help to accelerate the long-running process of relocating PMOI members outside Iraq, which has been going on for years.

Ban "condemns the attack... on Camp Hurriya (Liberty), near the Baghdad International Airport, which left at least 26 residents dead and many more wounded," a UN statement said.

He called for the perpetrators to be brought to justice and for the relocation process to be accelerated, terming it "the only safe and durable solution for the residents".

According to Iraqi security sources, at least 15 rockets fired from an area west of Baghdad called Bakriya struck in and around Camp Liberty on Thursday evening, while the PMOI put the number of rockets at more than 80.

Powerful Shiite militia groups are present in areas west of Baghdad, and the Islamic State jihadist group lacks the interest to attack the PMOI.

The PMOI released photos said to show bodies of the victims killed in the rocket attack, one of which showed 20 people lying in two rows on stretchers on the ground.

Camp Liberty, a former US military base, has since 2012 housed members of the PMOI, a group that originally opposed the shah but later fought alongside Saddam Hussein's forces against Iran's clerical rulers after the 1979 Islamic revolution.

The PMOI was also accused of taking part in the brutal suppression of a 1991 Shiite uprising against Saddam, making it widely reviled by members of the country's Shiite majority, which came to power after 2003.

- Iraq investigating attack -

Human Rights Watch has cited former PMOI members as having "reported abuses ranging from detention and persecution of ordinary members wishing to leave the organisation, to lengthy solitary confinements, severe beatings and torture of dissident members".

Iraqi joint operations command spokesman Yahya Rasool said an investigation into the attack "is under way", and that two Iraqi security personnel were wounded in the incident.

Rasool said a truck mounted with rocket launch tubes was found in Bakriya, north of Camp Liberty.

"The people behind this attack are terrorist criminals who want to destabilise the country," he said, without naming any group or nation.

US Secretary of State John Kerry said in a statement that "the United States strongly condemns (the) brutal, senseless terrorist attack on Camp Hurriya that killed and injured camp residents".

"No matter the circumstances, on this point we remain absolute: the United States remains committed to assisting the UN High Commissioner for Refugees in the relocation of all Camp Hurriya residents to a permanent and safe location outside of Iraq," he said.

The UN's High Commissioner for Refugees Antonio Guterres said: "This is a most deplorable act, and I am greatly concerned at the harm that has been inflicted on those living at Camp Liberty.

"Every effort must continue to be made for the injured and to identify and bring to account those responsible."

The PMOI has been repeatedly targeted in the years after being disarmed following Saddam's overthrow, with dozens of its members killed in attacks it generally blamed on Iranian and Iraqi authorities.

The group, which has a powerful network of former and current officials who advocate on its behalf, successfully lobbied to be removed from the US and EU terror lists, but more than 2,000 remaining dissidents at Camp Liberty are still waiting to be relocated to other countries.