The man who killed three people in a knife attack in a French church on Thursday morning is believed by police to be a 21-year-old Tunisian man who entered the country via Italy. His identity is still being checked by law enforcement.

French President Emmanuel Macron visited the scene of the attack in which several others were also wounded in the southern city of Nice.

In a speech, Macron said that he would be stepping up the deployment of soldiers to protect key French sites, such as places of worship and schools.

Two people – a woman and a church officer – were killed inside the Notre-Dame church while one died after seeking refuge at a nearby bar, AFP reported. According to the police on site, one of the victims, an elderly woman, was decapitated by the attacker. None of the victims has so far been named.

The attacker, a man, was allegedly shouting "Allahu Akbar" ("God is great"), as reported by Reuters. 

The city's mayor Christian Estrosi confirmed on Twitter that the police have arrested the attacker and that the suspect is currently in hospital after being shot by police while being detained. 

There were two further incidents on Thursday within hours of the Nice attack. A man was shot dead by police in Montfavet near Avignon, in the south of France, after threatening passers-by and police with a handgun. In the Saudi Arabian city of Jeddah, a knife-wielding man was arrested after attacking and wounding a security guard at the French consulate.

It's not clear yet whether the incidents are connected.

France's national anti-terrorist department has launched an investigation into the Nice attack, which Estrosi described as suspected terrorism.

"Enough is enough," the mayor told reporters. "It's time now for France to exonerate itself from the laws of peace in order to definitively wipe out Islamo-fascism from our territory."

All churches and religious centers in the city will now be placed under surveillance or closed.

Interior minister Gerald Darmanin said on Twitter that he had called a crisis meeting after the attack.

Prime minister Jean Castex said that the government's response will be "firm and implacable." France has moved up to its highest security alert level after the attack.

Only two weeks ago 47-year-old history teacher Samuel Paty was beheaded in a suburb of Paris after showing cartoons of the Prophet Muhammad to his students. Macron's response to the murder of Paty sparked anger and protests across the Muslim world.

There have been clashes between Macron and Turkish President Recep Tayyip Erdogan, but today Turkey has been one of the many nations expressing solidarity with France and condemning the attack. Turkish presidential communications director Fahrettin Altun said that Islam cannot be used in the name of terrorism.

"We call on the French leadership to avoid further inflammatory rhetoric against Muslims and focus, instead, on finding the perpetrators of this and other acts of violence," Altun wrote on Twitter.