According to Iraqi security sources, at least 15 rockets fired from an area west of Baghdad called Bakriya struck late yesterday in and around Camp Hurriya, also known as Camp Liberty.
It was not clear who was behind the strike.
"The Secretary-General condemns the attack yesterday on Camp Hurriya, near the Baghdad International Airport, which left at least 26 residents dead and many more wounded," Ban Ki-moon's spokesman said in a statement.
The PMOI sided with Saddam Hussein's Iraq during the war with Iran in the 1980s, but the 2003 US-led invasion brought to power leaders who despise the group.
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It successfully lobbied to be removed from the US terror list, but the more than 2,000 remaining dissidents at Camp Liberty are essentially stranded and demanding relocation.
Ban urged the Iraqi government and international stakeholders to work with the UN special advisor on the relocation of Camp Hurriya residents outside Iraq, as "the only safe and durable solution" to their plight.
"The Secretary-General calls on the Government of Iraq to promptly investigate the incident and bring the perpetrators to justice," said the UN statement.
"There can be no impunity for such attacks.