A year after President Obama stepped back from a bombing campaign in Syria, US officials are giving new thought to airstrikes by fighters, bombers, and unmanned drones.

WASHINGTON — The Obama administration is debating more robust intervention in Syria to break the back of the extremist group that has destabilized neighboring Iraq and killed an American, including possibly direct US military action, officials said Friday.

A top security adviser to the president vowed Friday that the United States will “do what is necessary” in Syria to protect American interests and said that direct military action was possible against the Islamic State militant group.


Benjamin J. Rhodes, the deputy national security adviser, said the Islamic State had become an increased threat to the United States, a threat the government takes seriously.

“If you come against Americans, we are going to come after you,” Rhodes said.

He declined to say whether the president was considering expanding airstrikes to include Islamic State targets in Syria as well as in Iraq, where raids began this month.

“We’re actively considering what’s going to be necessary in dealing with that threat,” Rhodes said. “We’re not going to be restricted by borders.”

General Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, said Thursday that the only way to defeat the Islamic State was for the United States or its allies to take the fight to the militants inside Syria.

“This is an organization that has an apocalyptic end-of-days strategic vision that will eventually have to be defeated,” Dempsey told reporters. “Can they be defeated without addressing that part of the organization that resides in Syria? The answer is no.”

Rhodes, speaking on Martha’s Vineyard, where the president is vacationing, said repeatedly Friday that the administration will do what was necessary in Syria to protect US interests, using language similar to Obama’s when he announced the airstrikes in Iraq.


“The American people understand that this president is very deliberate about military action,” Rhodes said. “American people also understand that there are some threats that have to be dealt with.”

Rhodes declined to say whether the administration would have to seek additional legal authority to conduct airstrikes or other military action inside Syria. He promised that the president and his staff will consult with Congress.

“We will take whatever action is necessary,” Rhodes said. “We will take direct action against terrorists that threaten the United States.”

The death toll from three years of Syria’s civil war has risen to more than 191,000, the United Nations reported Friday.

The figure, covering the period from March 2011 to April 2014, is the first issued by the UN’s human rights office since July 2013, when it documented more than 100,000 killed.

The high toll is a reflection of the brutality of Syria’s conflict, which has transformed into a complex, multilayered war where various factions fight against one another.

It also reflects the recent surge in deadly attacks by the Qaeda-breakaway Islamic State group targeting rival militant groups, mainstream Western-backed Syrian rebels, and Kurdish militiamen in northern Syria as it seeks to eliminate opponents and consolidate its hold.

Navi Pillay, the UN’s top human rights official, said the new figures are so much higher because they include additional killings from earlier periods, as well as deaths since the last report. The exact figure of confirmed deaths is 191,369.


‘‘Tragically it is probably an underestimate,’’ she said.