The president of the United States, Barack Obama, says the world is seeing a new phase of terrorism carried out by lone actors and small groups.
After a meeting with counter-terrorism chiefs Mr Obama said the smaller, self-initiating groups were harder to detect.
However, in a live television address he reassured Americans that as the threat changed, so counter-terrorism measures would evolve and improve.
"Anyone trying to harm Americans need(s) to know that we are strong and that we are resilient, and that we will not be terrorised.
"We have prevailed over much greater threats than this - we will prevail again."
Mr Obama said US intelligence professionals did not have any specific or credible information about a pending attack on the United States.
Speaking to reporters during a visit to advisers at the National Counterterrorism Center outside Washington, he warned the public to remain vigilant over the holiday season.
"When terrorists pull off a despicable act like what happened in San Bernardino, it tears at our hearts, but it also stiffens our resolve to learn whatever lessons we can and to make any improvements that are needed," he said.
Mr Obama again noted that the White House had taken significant measures to protect the United States since the September 11, 2001 attacks, although he added that it was natural for Americans to be anxious after the deadly incidents in Paris and California.
Mr Obama has made a series of statements in recent weeks to reassure Americans about the US security situation, including an address to the nation from the Oval Office earlier this month.
Republicans have criticised the Obama administration for not doing enough to counter the threat of Islamic State militants.
- BBC, CNN