President Obama condemned the multiple terror attacks in Paris late Friday, calling them an "outrageous attempt to terrorize innocent civilians" and vowing the U.S. will do what it takes to help "bring these terrorists to justice."
"This is an attack on all of humanity and the universal values that we share," Obama said from the White House. "... This is a heartbreaking situation."
While stressing he does not know all the details and does "not want to speculate," Obama said the U.S. is "prepared and ready" to give whatever assistance is needed to the French government and people.
"France is our oldest ally," Obama said.
The attack, and the president's comments, came hours after an interview aired in which Obama claimed the Islamic State was being contained.
"I don't think they're gaining strength," Obama told ABC News' "Good Morning America." "We have contained them. They have not gained ground in Iraq. And in Syria, they'll come in, they'll leave, but you don't see this systematic march by ISIL across the terrain."
He acknowledged coalition forces have not been able to "decapitate their command-and-control structures."
There was no immediate claim of responsibility for Friday's attacks. Online accounts linked to the Islamic State are celebrating the attack, though this does not represent a claim of responsibility.
More than 150 people were believed to have been killed in Friday's attacks, with gunmen executing scores of victims inside a Paris concert hall after killing dozens of others at separate locations. The State Department said that 70 U.S. citizens known to be in France have not yet been accounted for, though none of them has been reported killed.
The State Department says U.S. citizens can contact 1-888-407-4747 (from the U.S.) or 202-501-4444 (from other countries) for assistance.
Secretary of State John Kerry, saying he shares the president's "outrage and sadness," also said the U.S. embassy in Paris is "making every effort to account for the welfare of American citizens in the city."
"These are heinous, evil, vile acts," Kerry said. "Those of us who can must do everything in our power to fight back against what can only be considered an assault on our common humanity."
The series of attacks gripped the city in fear and recalled the horrors of the Charlie Hebdo carnage just 10 months ago.
A U.S. government official told Fox News they believe this was a coordinated attack. One source said it points to "months of planning" based on the multiple locations where the attacks were playing out.
Amid the attacks, Fox News is told that officials were "beefing up" security at the U.S. Capitol Friday evening, though there is no concrete intelligence indicating a threat in the U.S.
On Twitter, House Speaker Paul Ryan said, "All of Paris needs our prayers tonight."
The Department of Homeland Security put out a statement saying they are "closely monitoring" the events.
"At this time, there is no specific or credible threat to the United States," the DHS statement said.
According to a White House official, Obama has been briefed on the situation by Lisa Monaco, assistant to the president for homeland security and counterterrorism.
Obama said Friday the American people "stand together with [the French people] in the fight against terrorism and extremism."
"We're going to do whatever it takes to work with the French people [and other nations] ... to bring these terrorists to justice," he said.
Defense Secretary Ash Carter also has been briefed on the attack, according to Pentagon Press Secretary Peter Cook.
"At this point in time we are not aware of any DoD personnel involved in this tragedy. Our thoughts and prayers are with the French people at this difficult moment," Cook said.
The attack comes as France has heightened security measures ahead of a major global climate conference that starts in two weeks, out of fear of violent protests and potential terrorist attacks.
Fox News' Catherine Herridge and Chad Pergram and The Associated Press contributed to this report.