UN Secretary-General Ban Ki-moon and British Prime Minister condemned the "horrific attack" in Orlando in the US state of Florida, in which at least 50 people were killed and 53 were left injured.
The secretary-general "extends his deepest condolences to the families of the victims and expresses his solidarity with the government and people of the US," Xinhua quoted a statement as saying.
British Prime Minister also David Cameron expressed his horror by the shooting attack.
"I'm horrified by reports of the overnight shooting in Orlando. My thoughts are with the victims and their families," he tweeted.
Philip Hammond, the Secretary of State for Foreign and Commonwealth Affairs, also tweeted, "Shocked by shooting in Orlando". He noted that the Foreign and Commonwealth Office is in contact with the local authorities, adding, "Thoughts are with victims and families."
A gunman wielding an assault-type rife and a handgun killed at least 50 people in a nightclub in Orlando on Sunday in the worst mass shooting in the US history.
US President Barack Obama decried the tragedy as an "act of terror."
Islamic State-linked news agency, Amaq, said the attack was carried out by an Islamic State fighter.