President Barack Obama addressed questions that the United States had underestimated the threat of Islamic State attacks, noting that the US and its partners had “acute awareness” of the threat, but that its difficult to strike a non-governmental group.

"The concerns about potential ISIL attacks in the West have been there for over a year now and they come through periodically," Obama told reporters at a Group of 20 summit in Turkey on Monday, using an acronym for Islamic State (IS, also formerly known as ISIS).

The president promised that the US has not underestimated the ability of IS to strike Western countries militarily, but because the terrorist group does not conduct "conventional warfare," it is difficult to stop.

"We play into the ISIL narrative when we use routine military tactics that are designed to fight a state that is attacking another state,” he said. That's not what's going on here. These are killers with fantasies of glory who are very savvy when it comes to social media and are able to infiltrate the minds not just of Iraqis and Syrians but disaffected individuals around the world."

"It's not their sophistication or the particular weaponry they possess but it is the ideology that they carry with them and their willingness to die," Obama added.

There was no way to prevent the coordinated attacks in Paris on Friday night, Obama said.

"There were no specific mentions of this particular attack that would give us a sense of something that we could provide French authorities, for example, or act on ourselves," he told reporters.

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Obama has been on the defensive since Friday because of comments he’d made a day earlier claiming that IS has been “contained.”

“I don't think they are gaining strength,” Obama told ABC News' George Stephanopoulos in an exclusive interview on Thursday. “From the start, our goal has been first to contain, and we have contained them.”

Republicans have called on the president to send more ground troops to Iraq and Syria to battle IS, a possibility Obama dismissed on Monday.

"It is not just my view but the view of my closest military and civilian advisers that that would be a mistake," he said. "Not because our military could not march into Mosul or Raqqa or Ramadi and temporarily clear out ISIL but because we would see a repetition of what we’ve seen before, which is if you do not have local populations that are committing to inclusive governance and who are pushing back against ideological extremes, that they resurface, unless we’re prepared to have a permanent occupation of these countries.”

Critics, especially those vying for the GOP nomination, have also attacked the president for not doing enough to battle the threat of IS, but Obama noted that those who are complaining haven’t offered a viable solution.

"If there's a good idea out there, then we're going to do it," Obama said. "I don't think I've shown a hesitation to act."

If "folks want to pop off and have opinions about what they think they would do, present a specific plan," he added.

The president indirectly rebuked Ben Carson, the Republican frontrunner, for a comment last week in which the former neurosurgeon said that he had better intelligence on the ground in Syria than the White House does.

"If they think somehow that their advisers are better than my joint chiefs of staff or my generals on the ground, I want to meet them. And we can have that debate," Obama said.

He also took a jab at Donald Trump, another GOP candidate.

"What I’m not interested in doing is posing or pursuing some notion of American leadership or America winning or whatever other slogans they come up with that has no relationship to what is actually going to work to protect the American people and to protect the people in the region who are getting killed and to protect our allies and people like France,” Obama said. “I’m too busy for that.”

He also took Republicans to task for their calls to slam the door shut on Syrian refugees fleeing the violence wrought by IS in their home country, saying that "would be a betrayal of our values." Senator Ted Cruz (R-Texas), who is also running for president, has said that the US should bar Muslim refugees from Syria from entering the country, while still welcoming Christian refugees.

“That’s shameful, that’s not American, that’s not who we are. We don’t have religious tests to our compassion,” Obama responded on Monday. "Our nations can welcome refugees who are desperately seeking safety and ensure our own safety. We can and must do both."

At the same time, the president also called on Muslims to turn away from the violence and extremism that jihadist groups foment, adding that the religious fundamentalism they seek is the antithesis of true Islam.

“I do think that Muslims around the world…have to ask very serious questions about how did these extremist ideologies take root,” Obama said.