The United States is renewing pressure on its European NATO allies to establish a long-term train-and-advise mission in Iraq, diplomats said, reviving a divisive issue for an alliance wary after a decade in Afghanistan.

U.S. Secretary of Defense Jim Mattis sent a letter to NATO headquarters in January calling for a formal NATO mission to Iraq with a semi-permanent or permanent command to train Iraqi forces, according to five senior NATO diplomats.

While NATO does have trainers in Iraq already, they number less than 20. NATO defense ministers are expected to discuss the U.S. request in Brussels next week, with a possible decision at a summit in July.

In his letter, Mattis left many details open but suggested developing military academies and a military doctrine for the Iraqi defense ministry, diplomats said. Other ideas cited by diplomats include bomb disposal training, maintenance of Soviet-era vehicles and medical training.

"The United States is pushing hard for a NATO role in Iraq, not in a combat role, but for a long-term assignment," said one senior NATO diplomat on condition of anonymity, as reported by Reuters.

"This looks suspiciously like another Afghanistan," the diplomat said, referring to the long-running conflict where NATO is funding and training Afghan forces. "Few allies want that."

Iraq would need to formally request the NATO mission, diplomats said. That would likely rely on Iraqi Prime Minister Haider al-Abadi winning re-election in May, as rival candidates backed by Iran are hostile to U.S. troops remaining in the country.

A Pentagon report released in November said there were 8,892 U.S. troops in Iraq as of late September. Iraqi PM Abadi on Tuesday confirmed a fall in the number of U.S.-led coalition forces in the country but stressed the ongoing need for air support, according to AFP.

The U.S.-led coalition said on Monday it was "adjusting" its force levels in Iraq downwards as it shifts away from combat operations against Daesh. It gave no details but made clear the focus would now be on consolidating military gains against Daesh after more than three years of fighting in Iraq and Syria.

The Iraqi prime minister said the threat from Daesh was not over. "There is still a danger inside Syrian territory. We control the borders, but there could be a real danger," said Abadi. "We need significant air cover to monitor the desert and terrorist movements. We need efforts from the coalition and... want to do a fully successful job," he said.

In December, Iraq declared victory against Daesh, more than three years after the terrorist group seized a third of its territory and swathes of neighboring Syria, declaring a "caliphate" ruling over millions of people.