PARIS (AFP) - French President Francois Hollande and his United States counterpart Barack Obama vowed to reinforce their counter-terrorism cooperation following the attacks in Paris on Friday (Nov 14), according to a White House statement.

"The President reiterated the United States' steadfast, unwavering support for the people of France, our oldest ally and friend, and reaffirmed the offer of any necessary support to the French investigation," the statement said, after the pair had spoken on the phone. "The two leaders pledged to work together, and with nations around the world, to defeat the scourge of terrorism."

"Barack Obama wanted to express his support to the French people in the face of this terrible tragedy," said a source close to the French presidency. "They reaffirmed their commitment to working closely in the fight against terrorism."

Mr Obama also offered assistance to France even as officials said there's no specific threat to the US.

"We've seen an outrageous attempt to terrorize innocent civilians," Mr Obama told reporters in Washington. "This is an attack not just on Paris, an attack not just on the people of France, but this is an attack on all of humanity and the universal values we share."

At least 120 people were killed during multiple acts of violence in Paris, French prosecutors said. Many were hostages who had been taken captive at a concert hall.

US law enforcement officials are concerned that the attacks were well-planned, an American counterterrorism official said. There's nothing to suggest that an assault against the US is imminent, but France didn't appear to have any warning, said the official, who spoke on condition of anonymity.

If the Islamic State in Iraq and Syria (ISIS) is found to have carried out the Paris slaughter and the downing of a Russian airliner over Egypt two weeks ago, that would signal the group has evolved into a more sophisticated terrorist force that can carry out strikes beyond the Middle East, the official said.

The President was first informed about the attacks by his counterterrorism adviser Lisa Monaco, a White House official said on Friday. He still plans to travel to Turkey on Saturday for the G-20 summit.

Mr Obama had spoken earlier Friday, before the attacks, to French president Francois Hollande, who was evacuated from the stadium after the explosions. The two discussed climate talks in Paris, scheduled to begin at the end of the month, which Mr Obama plans to attend.

Mr Obama said the US would do whatever it takes to "bring these terrorists to justice and to go after any terrorist networks who go after our people".

He said the US did not yet know the details of what had happened, and it appeared there was still "live activity".

US officials have been in touch with counterparts in Paris, he said.

Mr Hollande declared a state of emergency and said he was tightening controls at France's borders. He also requested military reinforcement.

The US Department of Homeland Security and Federal Bureau of Investigation are closely monitoring events in Paris and know of no specific or credible threats of an attack on US soil of the type that occurred in the French capital, Homeland Security Secretary Jeh Johnson said in a statement.

For now, no new counterterrorism measures are planned at US airports or federal buildings, according to an American official, who discussed security planning on condition of anonymity.

The Paris attacks demonstrated a "relatively high degree of planning, casing and prep", said Mr Patrick Skinner, director of special projects for the Soufan Group, a New York security consulting firm.

"This wasn't a response to Jihadi John," Mr Skinner, a former Central Intelligence Agency case officer, said in an interview. "This took time."

US law enforcement and intelligence agencies are working in concert to determine if the attacks in France are part of an international terrorist campaign, said Representative Peter King, a New York Republican and member of the House intelligence committee, in an interview.

"This is the nightmare scenario for any large city, with a large population, these types of coordinated attacks," Mr King said. "This was undoubtedly carried out by Islamists, most probably" ISIS.

The US embassy in Paris was "making every effort to account for the welfare of American citizens in the city" and pledged that the US would provide whatever support France might require, said Secretary of State John Kerry, in a statement from Vienna where he is taking part in talks on Syria's civil war.

The New York City Police Department said it deployed officers to French government locations in New York City.

The department's intelligence bureau detectives in France are helping police there as needed, the NYPD said in a statement.

In Washington, US Capitol Police are monitoring events in Paris and there may be additional patrols, police spokesman Kim Schneider said in an e-mailed statement.

The French ambassador to the US Gerard Araud said in a Twitter message that he's "sad, devastated and anguished for my country".

The attacks came hours after the White House acknowledged the US had launched a strike targeting Mohammed Emwazi, an ISIS extremist known as "Jihadi John".

The ISIS leader became infamous internationally from his appearance in videos showing the beheadings of Americans held by the terrorist group.

The White House earlier Friday said the strike, in which Emwazi is presumed to have been killed, was evidence the US had made "important progress" in the fight against ISIS.

The President is scheduled to travel to Turkey on Saturday for the G-20 economic summit, where leaders say they will discuss efforts to combat ISIS.

Mr Obama plans to meet with Turkish president Recep Tayyip Erdogan on Sunday to discuss efforts to combat the Islamic State, according to deputy national security adviser Ben Rhodes.

Later that evening, G-20 leaders are scheduled to discuss the international campaign against the terror network.