France knife attack: World leaders stand united after 3 dead in ‘barbaric’ killings in Nice church

World leaders are standing united with France after a man armed with a knife decapitaed one woman and killed two others at a church in the southern city of Nice on Thursday.

French authorities have opened a terrorism investigation and Prime Minister Jean Castex said the country's threat level will be raised to its maximum after the attack.

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The suspect, believed to be acting alone, was injured during his arrest and was taken to a local area hospital. As he lay wounded, the Nice mayor said the attacker repeated “Allah Akbar!” over and over.

Ahead of French President Emmanuel Macron arrival in Nice later Thursday, leaders from around the world condemned the attack and expressed their unwavering support to France and its people.

President Trump said America stands with its oldest ally and called for an end to "radical Islamic terrorist attacks."

British Prime Minister Boris Johnson called the attack at the Notre Dame Basilica “barbaric,” tweeting in both English and French that the U.K. stands “steadfastly” with France.

"Our thoughts are with the victims and their families, and the UK stands steadfastly with France against terror and intolerance," Johnson said.

The Vatican in a statement said, “It is a moment of pain, in a time of confusion,” adding that “terrorism and violence can never be accepted.”

“Today's attack sowed death in a place of love and consolation, like the house of the Lord,” said Matteo Bruni, Director of the Holy See Press Office. “The Pope is informed of the situation and is close to the Catholic community in mourning.”

German Chancellor Angela Merkel said she was “deeply shaken by the terrible murders in a church in Nice,” adding that “the French nation has Germany’s solidarity in these difficult hours."

Turkey’s Foreign Ministry strongly condemned the attack in Nice amid its own heightened tensions with France.

“We stand in solidarity with the people of France against terror and violence,” the statement said.

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The energy-rich country of Qatar offered its “strong condemnation and denunciation” of the attack in France, saying in a statement that it wanted to reiterate its "firm position on rejecting violence and terrorism, regardless of the motives and reason.”

The French Council of the Muslim Faith condemned the Nice attack and called on French Muslims to refrain from festivities this week marking the birth of Muhammad “as a sign of mourning and in solidarity with the victims and their loved ones.”

The President of the European Parliament, David Sassoli, tweeted that “this pain is felt by all of us in Europe.”

“We have a duty to stand together against violence and those that seek to incite and spread hatred,” Sassoli said.

Spanish Prime Minister Pedro Sanchez also tweeted in both Spanish and French that we are “united in face of terror and hatred,” adding that Spain stands in “solidarity” with the victims of the attack.

Italian Prime Minister Giuseppe Conte said "we are united" following the "vile attack."

“The vile attack on the Cathedral of Nice will not weaken our common front in defense of the values ​​of peace and freedom,” Conte said. “Our convictions are stronger than fanaticism, hatred and terror. We are close to the families of the victims and to our French brothers.”

European Commission President Ursula von der Leyen was quick to condemn the “heinous” attack in France and pledged solidarity in Europe “in the face of barbarism and fanaticism.”

Danish Foreign Minister Jeppe Kofod condemned the deadly attack and said “Denmark stands shoulder to shoulder with our French friends and allies."

Lithuanian Foreign Minister Linas Linkevicius wrote on Twitter in French that “terrorism will never win over European values and our unity.”

Dutch Prime Minister Mark Rutte declared that the Netherlands stands with France against extremism.

“For the second time in a short time, France is startled by a gruesome act of terrorism, this time in Nice," Rutte tweeted. "Our thoughts go out to the next of kin. And we say to the French people: you are not alone in the fight against extremism. The Netherlands is next to you."

Meanwhile, Islamic State extremists issued a video on Wednesday renewing calls for attacks against France.

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France came under alert for Islamic extremist acts after an 18-year-old man of Chechen origin beheaded a French middle school teacher earlier this month after showing caricatures of the Prophet Muhammad in a civics lesson on free speech. 

A second incident on Thursday happened at the French Consulate in the Saudi Arabian city of Jiddah, where a Saudi man was arrested after stabbing a guard.

It wasn't immediately clear if that attack or the one in Jiddah involved the current controversy, which has seen protests across the Muslim world.

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A third incident on Thursday saw French police kill a man in southern France after he threatened a passerby with a handgun, Reuters reported, citing police. The man had reportedly shouted “Allahu Akbar.”

Fox News' Danielle Wallace and The Associated Press contributed to this report.