Obama says he cannot claim that we've been successful in Syria

Obama: Syria is one of the hardest issues I've faced

President Barack Obama said Friday he cannot claim the United States has been successful in Syria.

Obama, in his final news conference of the year, said he always feels responsible about issues happening around the world, but defended America's approach toward the civil war in Syria.

"With respect to Syria of what I have consistently done is taken the best course that I can to end the civil war and having also taken into account of the long-term national security interest of the United States," he said.

"Throughout the process based on hours of meetings, if you tally it up, days or weeks of meetings, where we went through every option in painful details and maps and with our military and we had our aid agencies and our diplomatic teams and sometimes we bring in outsiders who were critics of ours."

Obama told reporters he understood the desire for action to end the conflict, but it would have been impossible to do "on the cheap" without a full U.S. military intervention.

"Unless we were all in and willing to take over Syria, we were going to have problems," Obama said in the news conference, noting that it would have required "putting large numbers of U.S. troops on the ground, uninvited, without any international law mandate."

He also said that the world is united in horror at the assault on Aleppo, and that President Bashar Assad and his allies were responsible for atrocities including reported massacres of civilians.

"Responsibility for this brutality lies in one place alone: with the Assad regime and its allies Russia and Iran. And this blood and these atrocities are on their hands," Obama said.

— Reuters contributed to this report.