The attack in which a gunman opened fire at a packed nightclub in Orlando, Florida, is the worst mass shooting in US history. (Reuters Photo)
US President Barack Obama and Pope Francis on Sunday led the condemnation after a gunman went on the rampage at a gay nightclub in Florida leaving at least 50 people dead.
The pontiff condemned the killings as "homicidal folly and senseless hatred" while Obama described them as "an act of terror and an act of hate".
The attack in which a gunman opened fire at a packed nightclub in Orlando, Florida, is the worst mass shooting in US history.
Leaders from countries shaken by recent terrorist attacks, including France and Belgium, were among the first to express solidarity with the US.
Afghan President Ashraf Ghani sent his condolences to the families of the victims as authorities identified the alleged gunman as 29-year-old Omar Mateen, a US citizen of Afghan descent.
The FBI said Mateen had pledged allegiance to the ISIS group before the massacre which left 50 dead.
"I unequivocally condemn the horrific attack in #Orlando, #Florida, #USA. Nothing can justify killing of civilians," President Ghani tweeted.
Reacting to the latest in a litany of mass gun killings in the US, Obama said the FBI was "appropriately investigating this as an act of terrorism".
"Although it's still early in the investigation, we know enough to say that this was an act of terror and an act of hate," he said, later ordering flags at half-staff as an act of mourning.
In a statement from the Holy See, Pope Francis lamented the tragic loss of life.
The attack had "caused in Pope Francis, and in all of us, the deepest feelings of horror and condemnation... before this new manifestation of homicidal folly and senseless hatred", said the statement.
"We all hope that ways may be found, as soon as possible, to effectively identify and contrast the causes of such terrible and absurd violence," it added.
'Barbaric Crime'
In France, where the country remains on edge after ISIS terrorist killed 130 people in Paris in November 2015, French President Francois Hollande reacted "with horror".
The president "expresses the full support of France and the French people to the US authorities and the American people during this ordeal", a statement from the Elysee Palace said.
French Prime Minister Manuel Valls also expressed his "compassion" and "solidarity" with the American people.
"By striking the gay community, this terrible Orlando attack reaches us all," he wrote in French on Twitter.
Belgium's Prime Minister Charles Michel tweeted his condolences over the attack, which comes two-and-and-half months after dual strikes by jihadists on Brussels airport and metro that killed 32 people.
"Deeply saddened by the loss of so many innocent lives in #Orlando shooting. We join families in their grief," he said.
In Britain, Prime Minister David Cameron said on Twitter he was "horrified" by the shooting. "My thoughts are with the victims and their families," he said.
Israeli Prime Minister Benjamin Netanyahu, whose country saw four people shot dead by two Palestinian gunmen on Wednesday, also described the killings as "horrific".
"On behalf of the people and government of Israel I extend our deepest condolences to the American people following last night's horrific attack on the LGBT community in Orlando," he said in a statement issued in English.
"Israel stands shoulder to shoulder with the United States at this moment of tragic loss," Netanyahu added.
Wednesday's attack at a popular Tel Aviv nightspot triggered fresh Israeli restrictions on the West Bank and Gaza Strip at the start of the Muslim holy month of Ramadan.
NATO Secretary General Jens Stoltenberg urged nations to work together to reject such attacks.
"Terror and hate will not change who we are. NATO Allies stand united in the fight against terrorism and in defence of our open societies", he said on Twitter.
Russian President Vladimir Putin, meanwhile in a Kremlin statement, called the killings "a barbaric crime".