Washington, US (BBN)-The US has indicated a shift in its campaign against Islamic State militants in Iraq and Syria, including the use of direct ground raids.
Defence Secretary Ash Carter said there would also be more air strikes against "high-value targets", reports BBC.
Observers say his comments reflect acknowledgment of the lack of progress in defeating the militant group.
Separately, the US says Iran is being asked for the first time to international talks over Syria's war.
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.
"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," 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."
A US-led coalition began airstrikes against IS positions in Iraq and Syria last year. President Barack Obama said the objective was to "degrade and ultimately destroy" IS.
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 focussed on rebel opposition fighters and that it will only fuel more extremism.
Carter said the fight against IS would now concentrate mostly on Raqqa, the militants' declared capital in Syria, and Ramadi, the capital of Anbar province in western Iraq.
He did not divulge the circumstances by 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.
Meanwhile, US state department spokesman John Kirby said it was unclear if Iran's leaders would attend talks starting in Vienna this week.
Top envoys from the US, Russia, Arab and European nations are taking part.
Iran is Syrian President Bashar al-Assad's closest ally.
It is believed to have spent billions of dollars over the past four years to prop up his 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 Syria talks, it will now tolerate Tehran's involvement.
Why is there a war in Syria?
Anti-government protests developed into a civil war that, four years on, has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and al-Nusra Front, as well as less numerous so-called "moderate" rebel groups, who are strongest in the north and east. These groups are also battling each other.
What's the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad - including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.
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