The U.S. must continue its efforts to defeat the Islamic State of Iraq and Syria using a combination of military missions, crackdowns on illicit finance and counteracting propaganda, President Obama told reporters at the Pentagon Monday afternoon.

But it also means improving U.S. capabilities when it comes to preventing small-scale attacks.

"We are doing a better job at preventing large-scale attacks on the homeland. However, small lone-wolf attacks [and terrorist cells] have become hard to detect," Obama said. "That means we're going to have to pick up our game to prevent these attacks."

Still, Obama said U.S. airstrike, online, economic and political campaigns against the Islamic State have resulted in real strategic gains against the terror group.

"ISIL's strategic weaknesses are real," Obama said at a briefing where he was joined by his Joint Chief of Staff and Secretary of Defense. Obama spent the afternoon at the Pentagon discussing the current state of affairs against the Islamic State with those military leaders and about 40 top advisors for the Middle East and Africa.

When asked about the possibility of new U.S. forces in addition to the 450 he announced earlier this year, Obama stressed that the fight against the Islamic State won't be won by the U.S. alone. He also said it wouldn't happen quickly.

"This is a long term fight against [the Islamic State,]" Obama said. "If we try to do everything ourselves ... we'll be playing whack-a-mole."

Obama said the Islamic State has lost one quarter of the territory it controlled before coalition airstrikes began last year. Attacks have killed thousands of ISIS fighters, including senior commanders, and the effort has managed to bring countries together.

But House Armed Services Committee chairman Rep. Mac Thornberry, R-Texas, used the president's appearance to say despite the gains made on the ground, "his strategy to defeat ISIL isn't working."

"From Libya and Tunisia, to Afghanistan, [the Islamic State] continues to advance while we lose ground and time," he said. "I hope that the president will acknowledge these realities, end the veto threats on bills that would enhance his ability to take the fight to [the Islamic State] and rethink his own inadequate strategy."

The president used the opportunity to push the Senate to confirm Adam Szubin to be the next Treasury undersecretary for terrorism and financial crimes in order to allow the U.S. to go after terror financing, and also called on the Syrian people to unite against ISIS and transition to a new government that does not involve President Bashar al-Assad.