US Defence Secretary James Mattis (Photo: AFP)
The Pentagon was due to present the White House on Monday with possible new battle plans to defeat the Islamic State group, after President Donald Trump demanded top brass find additional ways to destroy the militants.
One of Trump's central campaign pledges was to quicken the fight against IS, and he berated the Barack Obama administration for taking too long to do so.
He claimed to have a secret plan to defeat IS, repeatedly pledged to "bomb the hell" out of the group and even threatened to kill family members of suspected IS fighters.
On January 28, Trump gave the Pentagon 30 days to come up with a comprehensive review of the fight against IS, which a US-led coalition has been bombing in Iraq and Syria since late summer 2014 while using commandos to train and advise local forces.
The review includes input from across the government, including its spy agencies, State Department and Department of Homeland Security.
While broad in scope, the options outlined in the review are preliminary and will be refined in the coming weeks, Pentagon spokesman Navy Captain Jeff Davis said.
"It's a plan to attack an enemy, and I don't think we are going to want to telegraph too much of it," Davis said.
The options presented to Trump will likely include more US troops being sent to the Middle East, and would see the Pentagon taking a more aggressive stance in other key areas.
"This is not about Syria and Iraq, it's about a trans-regional threat," General Joe Dunford, who is chairman of the Joint Chiefs of Staff, told a Washington audience last week.
"In this particular case we're talking about ISIS, but it's also Al-Qaeda and other groups that present a trans-regional threat."
He stressed the importance of renewed military action coming alongside diplomatic and political efforts.
"All of us who have participated in these conflicts over the last 15 years realize that anything we do on the ground has to be in the context of political objectives or it's not going to be successful," Dunford said.
Since the coalition effort to defeat IS began, the jihadists have lost much of the ground they once held and tens of thousands have been killed.
In Iraq, they are clinging to the last parts of their former bastion Mosul as Iraqi security forces backed by Western air power push them from the city.
The anti-IS fight is more complex in civil-war-torn Syria, where Russia is conducting its own air campaign to prop up President Bashar al-Assad.
A key question is whether America will arm Syrian Kurdish forces to lead the fight to retake Raqa -- a move sure to infuriate ally Turkey, which considers the Kurdish fighters terrorists -- or whether the United States should send in more combat troops.
Currently, only about 500 US troops are in Syria, mostly special operations forces working behind the front lines to train local Kurdish and Syrian Arab fighters.
Because Obama was elected in 2008 on the promise of ending US conflicts in the Middle East, he was loath to put US forces in combat.
The anti-IS review is also a significant first test for the relationship between Trump and his defense secretary, Jim Mattis.
Trump has so far given Mattis broad leeway on shaping US military policy, even though the retired Marine Corps general has pushed back against much of Trump's more blustery rhetoric, including his professed support of torture and claim that America should seize Iraq's oil.