Police have boosted security around sensitive landmarks in New York City after masked men attacked headquarters of satirical French magazine Charlie Hebdo in Paris, killing 12 people.

Counterterrorism patrols and additional police cars were deployed throughout the city, including at the French consulate and several news organisations.

New York Police Commissioner Bill Bratton said on Wednesday that the attack in France was “very concerning”, but said New York law enforcement agencies were not aware of any direct threat to the city.

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“These are not amateurs,” Bratton said, referring to the attackers in Paris. “They are quite clearly well-trained professionals on a mission.” New York City Mayor Bill de Blasio called it an attack free speech.

“It was an attack on the news media. It was an attack on freedom of expression. It was an attack on the values we hold dear,” de Blasio said before requesting a moment of silence at a police swearing-in ceremony in the city.

Authorities have also stepped up security at subways and train stations; national guardsman and police forces were sighted at Grand Central Terminal on Wednesday, local media reported.

The Paris attack follows a stand-off in Sydney, Australia, in December that ended when police stormed the cafe where a lone gunman had taken people hostage. Two hostages and the gunman were killed.

John Miller, deputy commissioner of intelligence and counterterrorism of the New York Police Department, said that such an attack was “inevitable” in his city.

Miller told CBS News it was “less a question of if, and more a question of when.” On October 23, a man attacked four New York City police officers with a hatchet. Zale Thompson, the assailant, had visited Islamic State and al-Qaeda sites more than 200 times in the days prior to the attack, according to New York police.

Canada also experienced deadly attacks carried out by men who officials there said were inspired by Islamic State militants who have taken over a large part of north-western Iraq and also are active in Syria.

Canada, France and Australia are among the countries that have joined a US-led air campaign to fight the militants.