WASHINGTON (AFP/REUTERS) - US President Barack Obama voiced renewed determination to destroy the ISIS on Monday (Dec 14), vowing to win back territory in the Middle East and kill the group's leaders.

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

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

"As we squeeze its heart, we will make it harder for ISIS to pump the propaganda to the rest of the world," the US leader insisted at the Pentagon after meeting the country's military leaders.

Listing a series of ISIS leaders killed in coalition operations, Mr Obama issued a stern warning.

"ISIL leaders cannot hide," he said, using another acronym for ISIS. "Our next message to them is simple, you are next."

The coalition was also targeting ISIS oil tanker trucks, wells and refineries. "We are hitting ISIL harder than ever," he said.

Mr Obama, a Democrat, has come under criticism by Republicans for not doing enough to counter Islamic State, in particular since the Nov 13 attacks in Paris that killed 130 people and were claimed by Islamic State, and the Dec 2 shooting in San Bernardino, California. The authorities believe the couple who killed 14 people in that attack were inspired by Islamist militants.

The White House has sought to counter those critics by outlining progress made since ISIS' rapid rise in Iraq and Syria more than a year ago.

As was the case in his primetime Oval Office address a week ago, Mr Obama offered no shift in policy, but admitted: "We are recognising that progress needs to keep coming faster."

He noted that ISIS had lost swaths of territory they once controlled in Iraq and Syria but that progress needed to be made faster against the group.

Mr Obama said he had tasked Defence Secretary Ash Carter with going to the Middle East to secure more military contributions for the coalition fight against ISIS.