|File photo of US fighter jet after carrying out air strikes in Syria, Sept 2014 AFP|
The US has indicated a shift in its campaign against Islamic State (IS) militants in Iraq and Syria, including the use of direct ground raids.
Defence Secretary Ashton Carter said there would also be more air strikes against "high-value targets".
Observers say his comments reflect acknowledgment of the lack of progress in defeating the militant group.
How long have the US and allies been bombing IS? A year now? And they are just realizing that it isn't working? Well, I just have to ask the question, "Was it supposed to work?" If it was, why did it take you a year to figure out that it wasn't? It must have been obvious; it was obvious to everyone else. Yet you kept it up with no serious change in plan. Is it any wonder why I question whether you actually intended to defeat IS?
A cynic might suggest that a continuous bombing campaign was good for American bomb manufacturers - keeps the inventory moving. Isn't that what most American military interventions are about? Well, good thing I'm not a cynic!
It was only after the immediate and dramatic success of the Russian bombing program that it became glaringly obvious that the American campaign was either a dismal failure or a sham. In any event, Americans do not like to be shown-up by Russians - not in hockey and not in war. So they have to do something dramatic like rescuing captives. Couldn't they have rescued captives a year ago?
There are a lot of questions to be answered about what the Americans have been doing in Syria and Iraq for the past year. I wonder if anyone other than me will ask them?
Separately, Iran says it is considering whether to attend international talks in Vienna this week on Syria's war.
Earlier, the US said Iran - an ally of Syria's President Bashar al-Assad - was being invited to the talks for the first time.
Foreign ministry spokeswoman Marzieh Afkham said "Iran's participation is under discussion", Iranian news agencies reported.
Mr Carter's comments, made to the Senate Armed Services Committee, come a week after US-Iraqi forces rescued dozens of hostages held by IS in Iraq.
Analysis by Jonathan Marcus, BBC defence correspondent
Russia's intervention in Syria has changed the military and diplomatic dynamic in the crisis and left Washington struggling to catch up.
Whatever the inconsistencies in Moscow's own policies, it has highlighted the deficiencies in Washington's approach - not least the collapse of its ailing train-and-equip programme for Syria that was largely going nowhere.
US Defence Secretary Ashton Carter wants a more active US strategy, but this inevitably runs counter to the prevailing mood in the White House.
Barack Obama, after all, has cast his presidency as one that will withdraw US troops from foreign wars, not engage in new ones. There is talk of deploying a small number of Apache attack helicopters to Iraq.
That could involve hundreds of extra US personnel. But US success still requires effective local allies on the ground and they are in short supply.
"We won't hold back from supporting capable partners in opportunistic attacks against ISIL or conducting such missions directly, whether by strikes from the air or direct action on the ground," Mr Carter said, using an alternative acronym for IS.
"We expect to intensify our air campaign, including with additional US and coalition aircraft, to target ISIL with a higher and heavier rate of strikes," he said.
"This will include more strikes against ISIL high-value targets as our intelligence improves."
|Free Syrian Army fighters (27 Oct 2015) AP|
The US says that it wants to work with partners including the Free Syrian Army
Although President Obama has not committed ground forces to Iraq, the US has about 3,500 troops in the country who have been helping to train Iraqi forces and also have a limited combat role.
Russia started its own air strikes in Syria at the end of last month, saying it wants to help President Bashar al-Assad defeat IS and other extremists.
But Washington has strongly criticised the Russian campaign, arguing that it has been focused on rebel opposition fighters and that it will only fuel more extremism.
|Russian air strikes have enabled President Assad's land forces to advance|
A Syrian tank fires in Harasta, northeast of Damascus, Syria (22 Oct 2015) AP
He did not divulge the circumstances under which the US might carry out operations on the ground on its own.
"[But] once we locate them, no target is beyond our reach," he said.
Foreign ministers from the US, Russia, Saudi Arabia and Turkey are due to meet in Vienna on Thursday evening, a Russian diplomatic source said, with Iran possibly joining further talks on Friday.
Friday's talks could also include officials from Egypt, Iraq and Lebanon.
Kremlin spokesman Dmitry Peskov called for a "widening of the dialogue" on Syria, when he spoke to reporters on Wednesday.
Iran is believed to have spent billions of dollars over the past four years propping up President Assad's government, providing military advisers and subsidised weapons, as well as lines of credit and oil.
It is also thought to have been influential in the decision of Lebanon's Hezbollah movement to send fighters to Syria to assist pro-Assad forces.
Syria's Western-backed opposition and the US's Gulf Arab allies have long opposed Iran's role in the Syrian war.
BBC diplomatic correspondent James Robbins says that while Washington is certainly not welcoming Iran to the Syria talks, it will now tolerate Tehran's involvement.