Accelerating anti-IS air war requires caution, says US general

The head of the US Air Force urged caution in increasing the pace of the aerial military campaign against the Islamic State group in Iraq and Syria on Tuesday, despite President Donald Trump's eagerness to battle the militant group.

General David Goldfein warned airstrikes must remain synchronised with the progress of coalition-backed local ground forces, as well as with political and diplomatic efforts.

Goldfein also 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.

President Donald Trump made accelerating the fight against IS a central plank of his campaign and has pledged to intensify bombing of the militant group in an alleged "secret plan".

In late January, Trump ordered generals to begin a 30-day review of the US strategy to defeat the Syria and Iraq-based militant group.

Trump also called for the "identification of new coalition partners" - a likely nod toward Russia.

After substantial territorial gains, IS is now on the back foot, struggling to hold onto the Iraqi city of Mosul and with its "capital" in Raqqa under threat.

But the battle is approaching a fork in the road.

Trump has reportedly shelved his predecessor Barack Obama's plans for taking Raqqa with the help of Kurdish forces and must soon decide how to proceed.

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

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