US considers allowing 'more civilian casualties' in anti-IS fight

US military chiefs are considering loosening restrictions on airstrikes against the Islamic State group in Iraq, raising the "acceptable" number of estimated collateral civilian casualties to authorise an attack.

Military advisors close to Defence Secretary James Mattis have proposed revising the rules that the Obama administration kept in place and giving American commanders a freer hand in ordering strikes in Mosul, The Washington Times reported.

Mattis' team is believed to be weighing the matter, with the expectation that the Trump White House would support such a move, one source, who recently left a high-level position in the department, said.

Another Pentagon official, who spoke on the condition of anonymity, told The Washington Times that the department has not drawn up any specific plan, but could not rule out that such a high-level policy change was being considered.

"I can tell you that isn't something that we are working on right now, but that's not to say something like that won’t happen," the official said.

President Trump has repeatedly said IS "must be eradicated from the face of the earth".

In the closing stages of his presidential campaign, Trump had repeatedly mocked the ongoing Mosul operation.

Trump said that former President Barack Obama's generals had flagged up the siege four months in advance, giving IS a warning on the assault.

"What a group of losers we have," Trump said, "They should have kept their mouths shut."

During the height of the Iraq War, the number of acceptable civilian casualties resulting from US airstrikes hovered around 30. But that number tends to fluctuate, depending on the target value, time sensitivity and other mitigating circumstances, military officials said.

The reports of potentially changing the rules of combat come as Human Rights Watch this week urged parties to minimise civilian casualties after an 'unlawful' air raid near Mosul believed to be carried out by US coalition or Iraqi forced killed one civilian and wounded 11 others.

On Wednesday, the top US ground commander in Iraq said there had been no changes to the rules of engagement for American forces, including air operations, but did not rule out modifications later on.

"Our mission has not changed, and we're operating under the exact same set of authorities [and] our orders here have not changed since the 20th of January," Army Maj. Gen. Joseph Martin told reporters at the Pentagon.

"I'm not going to be able to speculate on what the future will hold," he added during a briefing from Baghdad.

Gen. Martin acknowledged a surge in airstrikes in Mosul as Iraqi forces pushed into the IS-held western half of the city. But the general said intensified fighting was part of the three-month campaign and was unrelated to Trump moving into the Oval Office.

"We will conduct the fight the same way we've conducted the fight from the beginning" until ordered otherwise, he said.