US President Barack Obama has made a rare Oval Office address on the San Bernardino shootings that left 14 dead.
He said the killings were "an act of terrorism" designed to kill innocent people.
But "freedom is more powerful than fear," said Mr Obama, warning that divisiveness in American society would play into the hands of extremists.
He also said the US must make it harder for potential attackers to obtain guns.
Mr Obama vowed that the US would overcome the evolving threat of terrorism, but warned that Americans "cannot turn against one another by letting this fight be defined as a war between America and Islam".
"If we're to succeed in defeating terrorism, we must enlist Muslim communities as some of our strongest allies, rather than push them away through suspicion and hate," Mr Obama said.
He reminded Americans that Muslim-Americans were part of US society.
"And, yes, they are our men and women in uniform who are willing to die in defence of our country. We have to remember that," he said.
The president warned that turning against America's Muslim communities would be exactly what Islamist extremists in the so-called Islamic State (IS) group wanted.
Mr Obama said the US would draw upon "every aspect of American power" to combat IS.
"Our military will continue to hunt down terrorist plotters in any country where it is necessary," he said.
He added there were a number of things that could be done on home soil to combat terrorism.
He called for stricter gun control and said he had ordered the Departments of State and Homeland Security to review the visa waiver programme under which the female attacker in San Bernardino originally entered the US.
This was only the third Oval Office address of Mr Obama's presidency - they are reserved for events of national importance.
Mr Obama's speech was in response to a mass shooting by a married couple that left 14 dead.
The shooters, Syed Rizwan Farook, 28, and his wife Tashfeen Malik, 29, were killed in a shootout with police.
In his speech, the president characterised IS as "thugs and killers", adding: "The threat from terrorism is real, but we will overcome it."
The group said in a radio broadcast that the couple that carried out the attack were IS supporters, but gave no indication that IS was involved in its planning.
The FBI is also looking into reports Malik posted a message on Facebook pledging allegiance to IS around the time of the attacks.
Mr Obama has used Oval Office speeches sparingly compared with previous presidents. His previous two addresses, both in 2010, covered the Deepwater Horizon oil spill and the end of combat operations in Iraq.
Other significant Oval Office addresses include:
The Wall Street Journal said Mr Obama's use of today's speech to cover the terror threat "underscores how serious the issue has become for both the White House and the increasingly unsettled country".
FBI agents on Saturday raided a house in Riverside, California on a street where Farook used to live.
The home belongs to Enrique Marquez, a friend of Farook's who bought assault rifles used in the shooting but is not considered a suspect, US media reported.
Farook, who worked for the local health department, and Malik opened fire on an office Christmas party.
Their families have expressed astonishment at the attacks.
Farook's sister, Saira Khan, told CBS News: "I can never imagine my brother or my sister-in-law doing something like this, especially because they were happily married, they had a beautiful six-month-old daughter."
The couple used handguns and semi-automatic weapons that had been legally purchased in the US, police say.
Bomb equipment, weapons and thousands of rounds of ammunition were later found in their home.
It is the deadliest mass shooting in the US since 26 people were killed at a school in Connecticut in 2012.
The authorities said there was no indication so far the killers were part of an "organised group or formed part of a broader terrorist cell".