BAGHDAD - The Iraqi army has been paying salaries to at least 50,000 soldiers who don't exist, Iraqi Prime Minister Haider al-Abadi said on Sunday, an indication of the level of corruption that permeates an institution that the United States has spent billions on equipping and arming.

A preliminary investigation into so-called "ghost soldiers" - whose salaries are being drawn but are not in military service - revealed the tens of thousands of false names on Ministry of Defense lists, Abadi told parliament on Sunday. Follow-up investigations are expected to uncover "more and more" names, he added.

Abadi, who took power in September, is under pressure to stamp out graft that flourished in the armed forces under his predecessor Nouri al-Maliki. Widespread corruption has been blamed for contributing to the collapse of four of the army's 14 divisions in June in the face of an offensive by Islamic State extremists.

The United States is encouraging Abadi to create a leaner, more efficient military as the Pentagon requests $1.2 billion to train and equip the Iraqi army next year. The U.S. spent more than $20 billion on the force from the 2003 invasion until troops withdrew at the end of 2011.

With entry-level soldiers in Iraq drawing salaries of around $600 a month, the practice of "ghost soldiers" is likely costing Iraq at least $380 million a year - though officials say that's probably only a fraction of the true expense.

"It could be more than triple this number," said Hamid al-Mutlaq, a member of the parliamentary defense and security committee, pointing out that more thorough on-the-ground investigations are planned. "The people who are responsible for this should be punished. Iraq's safe has been emptied."

The corrupt practice is often perpetrated by officers who pretend to have more soldiers on their books in order to pocket their salaries, experts say.

Abadi's announcement was met with applause in parliament. He said that the perpetrators would be tracked down eventually, but his priority is to end the practice as soon as possible.

"We are losing money all the time," he said. Since taking power Abadi has also retired dozens of officers who served under Maliki, who has been widely accused of promoting based on loyalty rather than merit.

"The problems are wide and it's an extremely difficult task which is going to involve some strong will," said Iraqi security analyst Saeed al-Jayashi. "Training is weak and unprofessional."

The United States is focusing its efforts on three divisions, in order to begin effective counteroffensive operations against Islamic State, which controls around a third of the country's territory.

The Pentagon has also requested a further $24 million to train and equip tribal fighters, and $354 million for Kurdish forces as part of its strategy to turn the tide against the group.

Abadi admitted that the process of arming the Sunni tribes had been slow, but he promised support for all those fighting against Islamic State. "We do not want to just give weapons randomly," he said.