WASHINGTON (REUTERS) - The Islamic State in Iraq and Syria (ISIS) has claimed responsibility on Sunday for the worst mass shooting in US history, but US officials said they had seen no immediate evidence linking the militant group to the massacre in Orlando, Florida.

Islamic State's claims were carried by the group's Albayan Radio on Monday (June 13) as well as Amaq, the organisation's news agency, on Sunday. "One of the Caliphate's soldiers in America carried out a security invasion where he was able to enter a crusader gathering at a nightclub for homosexuals in Orlando, Florida ... where he killed and injured more than a hundred of them before he was killed," the group said in its radio broadcast.

The Amaq statement carried a similar message.

At least 50 people were killed and 53 others were wounded in the Pulse nightclub before the gunman was shot dead by police.

The shooter was identified by authorities as Omar Mateen, a Florida resident and US-born son of Afghan immigrants who a senior FBI official said might have had leanings towards ISIS.

The FBI official cautioned, however, that proving the suspected link to radical Islamism required further investigation.

Three US officials familiar with the investigation into the massacre said that no evidence had yet been found showing a direct link with Islamic State or any other militant group.

There is "no evidence yet that this was directed or connected to ISIS. So far as we know at this time, his first direct contact was a pledge of bayat (loyalty) he made during the massacre," said a US counter-terrorism official, referring to a 911 call the suspect made on Sunday. The official spoke on condition of anonymity.

A US intelligence official said it was not unexpected that ISIS would claim responsibility given that the group has been suffering serious losses of fighters and territory in Iraq and Syria.

"The fact that a website connected to Daesh applauded it doesn't mean anything," said the US intelligence official, using an Arabic language acronym for ISIS.

"They are losing on their home turf, and it's not surprising if they're looking for some kind of twisted victory."

Speaking at the White House, President Barack Obama called the attack "an act of terror and an act of hate" and said the FBI would spare no effort to determine whether the suspect had been inspired by any extremist group.

The two officials familiar with the investigation said a leading theory was that the suspect somehow was inspired by Islamist militants.

One official said early information, the nature of which he did not disclose, indicated that the shooter was motivated by a mixture of "hate" and religion.

ACT OF TERRORISM

US Representative Adam Schiff of California, the senior Democrat on the House Intelligence Committee, said in a statement issued after a briefing on the massacre that several factors indicated the attack was an Islamic State-inspired "act of terrorism".

He noted that the incident occurred during the Islamic holy month of Ramadan, that Islamic State had called for attacks during that period, the target was an LGBT nightclub and it was hit during Gay Pride weekend.

Schiff said that, if accurate, "according to local law enforcement the shooter declared his allegiance to ISIS (Islamic State)".

An audio message purportedly issued last month by the spokesman of Islamic State called on followers to launch attacks in the United States and Europe during Ramadan, which began on June 5 in the United States.

"Ramadan, the month of conquest and jihad. Get prepared, be ready... to make it a month of calamity everywhere for the non-believers... especially for the fighters and supporters of the caliphate in Europe and America," said the statement allegedly made by Abu Muhammad al-Adnani and distributed over Twitter accounts usually associated with Islamic State.

"The smallest action you do in their heartland is better and more enduring to us than what you would if you were with us. If one of you hoped to reach the Islamic State, we wish we were in your place to punish the Crusaders day and night," said the audio clip, the authenticity of which could not be verified.