'You are next', Obama vows to kill Islamic State leaders, destroy the terror group

US President Barack Obama voiced fresh determination to destroy the Islamic State on 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 extremists in Iraq and Syria, but admitted that progress needed to come faster.

"We are hitting ISIL harder than ever," said Obama, in a second address following the seemingly Islamic State-influenced attack in San Bernardino, California 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 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 group's proclaimed "capital" at Raqa.

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.

According to a Wall Street Journal/NBC poll published Monday, Americans now view national security as a top priority.

The same poll found that Obama's own job approval ratings were at the lowest level this year, at 43 percent.

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.

- Republican criticism -

Like his primetime Oval Office address a week ago, Obama on Monday offered no shift in policy on defeating the Islamic State group, admitting: "We recognize that progress needs to keep coming faster."

Obama has advocated a multi-pronged strategy of airstrikes, special forces operations, financial sanctions and diplomacy.

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

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' because we haven't been hitting them very hard." The issue is sure to feature prominently when Republicans take to the debate stage on Tuesday.

Hours before that, Hillary Clinton will set out her counterterrorism strategy, during a visit to Minnesota. According to aides the speech will outline her "strategy to confront the threat of domestic radicalization and foreign-inspired terrorist attacks inside the United States."