US President Barack Obama voiced fresh determination to destroy the Islamic State Monday, vowing to kill the group's leaders and win back territory in the Middle East.

Sounding a notably more strident tone, Obama said that the United States and its allies were taking the fight to Islamic State in Iraq and Syria.

"We are hitting ISIL harder than ever," said Obama, in a second address following the seemingly Islamic State-influenced attack in San Bernardino that has raised questions about his strategy.

"As we squeeze its heart, we'll make it harder for ISIL to pump its terror and propaganda to the rest of the world," Obama insisted at the Pentagon, after meeting with top military and national security advisors.

Listing eight Islamic State figures killed in coalition operations, Obama issued a stern warning.

"ISIL leaders cannot hide and our next message to them is simple: You are next."

Obama said that US special forces were now in Syria and were helping local groups squeeze the Islamic State's proclaimed "capital" at Raqqa.

Meanwhile, he said, Iraqi forces were moving to take Ramadi "encircle Fallujah and cut off ISIL's supply routes into Mosul."

From the air, Obama said the United States and its allies had begun targeting "oil infrastructure, destroying hundreds of their tanker trucks, wells and refineries."

"Since the summer, ISIL has not had a single successful major offensive operation on the ground in either Syria or Iraq," Obama said.

Even before the December 2 attack by a Muslim husband and wife in California killed 14 people, polls showed that more than 60 percent of Americans disapproved of the way Obama is handling the Islamic State and the broader terror threat.

That is a major shift since Obama's first term in the White House, when he was hailed for authorizing a high-risk special forces raid that killed Osama bin Laden.

But like his primetime Oval Office address a week ago, Obama on Monday offered no shift in policy, admitting: "We recognize that progress needs to keep coming faster."

Obama has advocated a multipronged strategy of airstrikes, special forces operations, financial sanctions and diplomacy aimed at making Syria less chaotic.

Obama -- with the failures of occupations in Iraq and Afghanistan seared into his political psyche -- has steadfastly ruled out deploying large numbers of infantry troops.

Republican presidential hopeful Jeb Bush said Obama's remarks were further evidence of a half-hearted strategy.

"We're only hitting ISIS 'harder than ever'" he said, because "we haven't been hitting them very hard."