WASHINGTON — President Barack Obama pledged Sunday that the United States would defeat a terrorist threat that has entered a "new phase," speaking to Americans in an attempt to reassure the country after attacks in Paris and California.

"I know that after so much war, many Americans are asking whether we are confronted by a cancer that has no immediate cure," Obama said during a 13-minute speech from a lectern in the Oval Office.

Watch President Obama's address to the nation on the threat of terrorism and keeping the American people safe. https://t.co/TPomO9NKs8 — WH National Security (@NSCPress) December 7, 2015

In the prime-time speech, Obama did not roll out major policy changes in the battle against the Islamic State or propose any major new domestic security initiatives. But he did say the United States can — and should — make it harder for would-be mass killers to obtain guns. He specifically called on Congress to prohibit people suspected of terrorism or those on the no-fly list from buying guns.

POTUS calls on Congress to pass bill to stop those on terror watch list from being able to buy guns. Thursday -> https://t.co/IZgkAE4UT9 — Frank Thorp V (@frankthorpNBC) December 7, 2015

He also called for "stronger screening for those who come to America without a visa," as Congress is moving to overhaul the so-called visa waiver program that allows travel to the U.S. with no visa. The program has come under scrutiny since the Paris attacks.

The speech comes four days after a husband and wife killed 14 people and wounded nearly two dozen more in San Bernardino, Calif. So far, investigators have released little information about the shooters, identified as Tashfeen Malik and Syed Rizwan Farook.

Obama said that it was clear that Malik and Farook had gone down the "dark path of radicalization."

"This was an act of terrorism, designed to kill innocent people," he said Sunday night, adding that the duo had stockpiled weapons and ammunition.

"So far we have no evidence that the killers were directed by a terrorist organization overseas” or part of broader plot at home, Obama added.

Sunday night's speech is just third time that Obama has given an address from the Oval Office. Obama's first Oval Office speech took place in 2010, when he announced a plan for the Deepwater Horizon oil spill. In his second prime-time address, which also took place in 2010, Obama announced the end of United States combat operations in Iraq.