US General David Petraeus, a former commander of US forces in Iraq, said Friday he would support targeted strikes against jihadists behind an offensive in the strife-torn country, who he said were developing into a "terrorist army".

Petraeus, who won praise for leading the troop surge that preceded Washington's exit from Iraq after a costly eight-year war, told the Daily Telegraph newspaper that the militants posed a risk to countries outside the region.

"We must be careful not to take sides if we offer military support. But the growing threat posed by ISIS (Islamic State of Iraq and the Levant, or ISIL) means that military action will be necessary," he said.

"We must realise that ISIS poses a threat not only to Iraq but to the UK and other countries as well."

He added: "It seems to be much more than a terrorist group: it seems to be turning into a terrorist army, one that has acquired vast financial resources from looting banks and other criminal enterprises."

US President Barack Obama on Thursday pledged to take "precise" military action if required in Iraq and offered up to 300 US advisors to train Iraqi forces, after the Shia-led government in Baghdad formally asked for air support.

"If President Obama and other leaders conclude that the threat posed by ISIS is significant then I would support actions to target high-value ISIS elements," Petraeus told the Telegraph.

"If ISIS is seen as a terrorist organisation with the potential to engage in terrorist acts beyond the Middle East, then that could warrant the targeting of high value targets."

In a speech in London on Thursday, however, Petraeus warned there needed to be a radical change of politics in Baghdad to reflect Iraq's multi-confessional, multi-ethnic make-up before any US intervention.

"This cannot be the United States being the air force for Shia militias, or a Shia on Sunni Arab fight," he said.