Defense Secretary Ashton Carter said Tuesday the United States will increase attacks on Islamic State jihadists in Syria and Iraq, with additional air strikes and even "direct action" on the ground.

Testifying before the Senate Armed Services Committee, Carter said he expects more actions like the one last week that freed dozens of captives but left an American commando dead 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.

The Obama administration opposes committing US ground forces to Syria, but currently some 3,500 US forces are in Iraq in a "train and advise" capacity to support local forces as they fight IS jihadists.

Carter's statement appears to be a doubling-down of comments he made last week following the raid in which US Special Operations forces and Kurdish peshmerga troops stormed an IS-run prison near Hawijah in northern Iraq, freeing some 70 captives who were facing imminent execution.

Following the raid, in which Master Sergeant Joshua Wheeler became the first American serviceman to die in action in Iraq since 2011, Carter said he expected "more of this kind of thing."

The defense chief also warned that more American soldiers would likely be in harm's way as a result. "This is combat and things are complicated," he said.

Defense officials say any additional Special Operations ground actions would fall under existing authorizations for the anti-IS fight.

Carter told senators the United States is focusing on the IS stronghold of Raqqa in northern Syria and will boost support for rebels fighting the jihadists.

"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," Carter said, using an alternate acronym for IS.

Additional Syria strikes will occur once the United States has better intelligence on IS targets.

- The three 'Rs' -

Extra raids and a focus on Raqqa are two components of an anti-IS strategy Carter described as being centered on the "three Rs" -- raids, Raqqa and Ramadi.

Ramadi is the capital of Iraq's Anbar province and has been held by IS forces since May. Local Iraqis, supported by US air power, are trying to retake it.

In Syria, the Pentagon is dropping ammunition to rebels in the north in a program that replaced a failed bid to build a rebel army to fight IS.

Defense officials hope recipients will pressure IS fighters in Raqqa.

Carter's pledge to intensify strikes comes as the US-led coalition has been hitting fewer targets in Syria.

Pentagon officials insist the diminished tempo reflects a lack of decent targets, and is unrelated to Russia launching its own bombing campaign a month ago.

More than 250,000 people have been killed since Syria's brutal conflict broke out in March 2011, sparked by a bloody crackdown on protests against the rule of President Bashar al-Assad.

Carter testified alongside General Joe Dunford, who is America's top officer and the chairman of the Joint Chiefs of Staff.

- Republicans react -

Their comments about Syria elicited howls of disapproval from Republican lawmakers, who say America should do more to protect civilians and support Assad's overthrow.

Russia is bombing for the Assad regime -- though Moscow claims it is targeting IS and other "terrorists" -- and Iran has sent advisers and troops to Syria to help the government.

Dunford said the "balance of forces" is currently in Assad's favor.

"Russia, Iran and Hezbollah are going to fight for their guy and we are not going to do a damn thing to help the people who want to change Syria for the better by getting rid of the dictator in Damascus," Senator Lindsey Graham said, calling the US approach "half-assed" at best.

Lawmakers asked why America hasn't established no-fly zones or protective buffer regions in Syria.

Carter and President Barack Obama are reluctant to undertake such a commitment due to massive costs and the risk to US personnel.

No-fly zones have only gotten more complex as a concept since Russia entered Syria's war a month ago.

Dunford said that on a visit to Baghdad last week, leaders had assured him they had not asked Russia to extend air strikes into Iraq.