Prime Minister Theresa May said on Monday that Britain's thoughts were with the victims of what she called an "appalling attack" in Las Vegas, where a gunman opened fire at a country music festival.

"The UK’s thoughts are with the victims and the emergency services responding to the appalling attack in Las Vegas," May said in a post on her official Twitter feed.

In the meantime, British Foreign Secretary Boris Johnson said he was horrified by an attack at a music festival in Las Vegas that left at least 50 people dead, and said the country stood ready to help however it could.

"I am horrified by the awful attack at a music festival in Las Vegas this morning," he said in a statement. "The United Kingdom stands with the American people against this indiscriminate violence. My thoughts are with all those caught up in it."

"The Foreign and Commonwealth Office is in contact with Las Vegas authorities to establish whether any British people were caught up in the attack and we are ready to help however we can."

Pope Francis as well has called the killing of more than 50 people by a gunman in Las Vegas a "senseless tragedy".

"Deeply saddened to learn of the shooting in Las Vegas, Pope Francis sends the assurance of his spiritual closeness to all those affected by this senseless tragedy," said a message sent to the Roman Catholic bishop of the Nevada city on Monday.