President Obama said the US is hitting ISIS "harder than ever" in a speech at the Pentagon on Monday, in the wake of criticism that he is not doing enough to combat the extremist group.

"ISIL leaders cannot hide, and our next message to them is simple: You are next", said Obama, listing the leaders of ISIS that have been killed by American air strikes, including Briton 'Jihadi John'.

Obama used ISIL, the US government's preferred term for ISIS, and said that militants had lost 40 per cent of the land they once held in Iraq.

In the month of November, the US launched more air strikes against the group than in any other month so far, he said. Since the beginning of the summer of 2014 around 9,000 air strikes have been launched against ISIS.

"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 said that even before the Paris and San Bernardino attacks, the campaign had been moving forward "with a great sense of urgency". However,  "progress needs to be coming faster," he said.

After his speech, Obama told reporters that the effort against ISIS "continues to be a difficult fight".

"ISIL is dug in, including in urban areas, and they hide behind civilians, using defenceless men, women and children as human shields," he said. "So even as we're relentless, we have to be smart, targeting ISIL surgically, with precision."

Obama spoke at the Pentagon after a meeting with his National Security Council, who he rarely meets outside the White House Situation Room. He said it was part of a plan to "review and constantly strengthen" American plans against ISIS.

Some Republican politicians have said they are not convinced by his strategy, however. House majority leader Kevin McCarthy said, "The American people are smart enough to know when something is working or not, and it's obvious that the president's current strategy isn't working."