The head of the US Air Force urged caution Tuesday in increasing the pace of the air war against the Islamic State (IS) militant group in Iraq and Syria.

President Donald Trump made accelerating the fight against IS a central plank of his campaign and has pledged to intensify bombing of the IS group.

General David Goldfein told journalists that air strikes must remain synchronized with the progress of coalition-backed local ground forces, as well as with political and diplomatic efforts.

And he warned against loosening rules of engagement that could result in greater numbers of civilian casualties.

"One of the things I am very proud of is that we have never lost sight of the fact that we go to war with our values," he said.

"Short-terms gains that may be accomplished by rapid increases in the number of weapons or numbers of sorties pale in comparison to long-term costs if we were to step away from how we fight, which is as a nation of values," he added.

Trump has given top military brass until the end of February to come up with a revised plan to defeat IS.

The US-led coalition has already carried out more than 18,000 air strikes since the beginning of the campaign in late summer 2014.

The Pentagon says at least 199 civilians have been killed, though critics say the real number is far higher.

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