Asserting that the ISIS does not speak for Islam, US President Barack Obama has urged Islamic leaders across the world to speak out against the terrorist outfit and its ideology.

In a rare address to the nation from his Oval Office, Obama also urged his countrymen not to discriminate against people based on religion, as being feared inside America in the aftermath of a radicalised couple killing 14 people in San Bernardino in California.

Obama described the incident as an "act of terrorism".

"So far, we have no evidence that the killers were directed by a terrorist organisation overseas or that they were part of a broader conspiracy here at home," Obama said referring to the two suspect Pakistani national Tashfeen Mailk, and her husband, Syed Rizwan Farook, of Pakistan origin.

"But it is clear that the two of them had gone down the dark path of radicalisation, embracing a perverted interpretation of Islam that calls for war against America and the West. They had stockpiled assault weapons, ammunition, and pipe bombs. So this was an act of terrorism designed to kill innocent people," Obama asserted.

In a passionate speech from the Oval Office, Obama urged Americans not to turn against one another by letting this fight be defined as a war between America and Islam. "That, too, is what groups like ISIL (another name for ISIS) want," he noted.

"ISIL does not speak for Islam. They are thugs and killers, part of a cult of death. And they account for a tiny fraction of a more than a billion Muslims around the world, including millions of patriotic Muslim-Americans who reject their hateful ideology," he said.

"Moreover, the vast majority of terrorist victims around the world are Muslims. 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," Obama said.

That does not mean denying the fact that an extremist ideology has spread within some Muslim communities. It's a real problem that Muslims must confront without excuse, he asserted.

"Muslim leaders here and around the globe have to continue working with us to decisively and unequivocally reject the hateful ideology that groups like ISIL and Al Qaeda promote, to speak out against not just acts of violence, but also those interpretations of Islam that are incompatible with the values of religious tolerance, mutual respect, and human dignity," Obama said.

But, just as it is the responsibility of Muslims around the world to root out misguided ideas that lead to radicalisation, it is the responsibility of all Americans, of every faith, to reject discrimination, he asserted.

"It is our responsibility to reject religious tests on who we admit into this country. It's our responsibility to reject proposals that Muslim-Americans should somehow be treated differently. Because when we travel down that road, we lose. That kind of divisiveness, that betrayal of our values plays into the hands of groups like ISIL," Obama said.