Havana, Jun 14 (IANS): Cuban President Raul Castro has condemned the Orlando mass shooting that left 50 people killed and 53 others injured, expressing solidarity with the families of the victims, the US people and government.

"The Cuban people and government express their condolences and solidarity with the families of the victims," Xinhua news agency quoted Castro as saying in a message to US President Barack Obama.

The letter, published by Havana's Foreign Affairs Ministry, is the first of its kind sent by Castro to Obama since the two nations re-established diplomatic relations in 2015.

"Cuba rejects and unequivocally condemns all acts of terrorism or hate anywhere, under any circumstances and whatever the motives may be," Castro said.

Afghan-origin Omar Mateen, 29, of Florida opened fire at the Pulse gay nightclub on Sunday around 2.00 a.m. (local time), making it the deadliest mass shooting in the US history after the attacks in 2011.

Islamic State militant group has claimed responsibility for the attack, but the extent of its involvement is being probed.

Dubbed Pride Month, June was widely celebrated among the LGBT communities. The nightclub Pulse was one of Orlando's most well-known venues that hosts nightly LGBT-themed performances as well as a monthly programme of LGBT-related educational events.

Obama, in a national address in Washington on Sunday afternoon, said the Orlando shooting was an "act of terror and an act of hate" by a "lone wolf".

He said authorities should spare no effort to determine whether the killer had any ties to extremist groups.

The White House on Monday called the shooting "homegrown extremism", and said it appeared that the shooter was motivated by extremist propaganda online as no evidence was found about his direct links with radical groups.

Mateen, an employee of a British security firm G4S, had been investigated by the Federal Bureau of Investigation in 2013 and 2014 for possible connection to extremists.