The gunman who shot at least 20 people in an Orlando nightclub might have had leanings toward Islamic State militants but this requires further investigation, a senior FBI official said.

When asked if the FBI suspected the gunman might have had inclinations toward militant Islam, Assistant FBI Agent Ronald Hopper said: "We do have suggestions that the individual may have leanings toward that particular ideology. But right now we can't say definitively."

Danny Banks, special agent in charge of the Florida Department of Law Enforcement said: "Do we consider this an act of terrorism? Absolutely, we are investigating this from all parties' perspective as an act of terrorism."

He added: "Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of."

President Barack Obama ordered the federal government to provide any assistance needed to Florida police investigating the shooting, the White House said in a statement.