Secretary of State John Kerry condemned the "murderous" attack on a Paris magazine Wednesday and sent a message to the assailants: "The world will never give in to the intimidation and the terror."
Three gunman burst into the offices of the satirical publication Charlie Hebdo and killed 10 journalists and at two police officers in the worst act of terrorism in France in recent years.
"I would like to say directly to the people of Paris and all of France that each and every American stands with you today," Kerry said in a statement that he also delivered in French.
"No country knows better than France that freedom has a price because France gave birth to democracy itself," he added, saying he agreed with the characterization of the victims as "martyrs for liberty."
U.S. officials said they are keeping tabs on threats toward Americans in the wake of the killings and were ready to assist France in any way.
"The Department of Homeland Security is closely monitoring the unfolding events in Paris and we remain in contact with our counterparts in the region," a DHS official said. "DHS will not hesitate to adjust our security posture, as appropriate, to protect the American people."
Pentagon officials say there has been no decision to increase the level of security for U.S. military bases and personnel throughout Europe. However, military personnel across the globe started taking precautions — such as wearing civilian clothes instead of uniforms off base — following strikes against ISIS and the terrorist killings of a British soldier in London and two Canadian soldiers in Ottawa.
The New York City Police Department said it has an officer stationed in Paris who is monitoring and relaying developments, but it stressed there is no direct threat to New York.
Members of Congress also issued expressions of solidarity with France.
"This event in Paris recalls to mind what we lived through not that long ago when the United States on September 11, 2001, was attacked by terrorists and more than 3,000 innocent Americans lost their lives in New York, in Washington, and on the countryside of Pennsylvania," Sen. Dick Durbin, D-Ill., said on the Senate floor.
"Many of us recall that at that moment, that sad, awful moment, that people around the world rallied to stand with the United States in our grief and in our determination for justice. We particularly remember that the people of France did that, and they spoke out in one voice saying that they were going to be by our side in this battle against terrorism.
"I think it is appropriate today that we follow suit, that we join in that same spirit."
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NBC News' Abigail Williams, Frank Thorp and Jim Miklaszewski contributed to this report.