Ted Cruz accused President Obama's administration of a "wilful blindness" to the threat of radical Islam at a Senate hearing on Wednesday.

The former Republican presidential hopeful said the Obama administration had under-emphasized the "threat of radical Islam". He pointed to the initial redaction of references to the leader of Islamic State in partial transcripts released by the FBI and US State Department of the 911 call made by Orlando shooter Omar Mateen.

They later reversed their decision and released the unredacted version after a wave of criticism.

Obama has refused to use the term "radical Islam" in reference to terror attacks. Presumptive Republican presidential nominee Donald Trump gave a national security speech a day after the Orlando massacre, insisting it was "time to tell the truth about radical Islam".

In his own remarks, Obama said America stood united in the face of "an act of terror and an act of hate" and would not give in to fear.

"In the face of hate and violence we will love one another," he added. "We will stand united as Americans to protect our people, defend out nation and take action against those who threaten us."

Administration officials insist that focusing on the use of the phrase 'radical Islam' is a political distraction and gives fuel to Islamic State propagandists who say the United States has waged war against all of Islam.

Absent from Tuesday's hearing before a subcommittee of the Senate Judiciary Committee were the two top Justice Department counter-terrorism officials whom Cruz had invited to testify.

"The Obama administration of course has declined to appear and explain its policies," Cruz said at the hearing, titled "Willful Blindness: Consequences of Agency Efforts To Deemphasize Radical Islam in Combating Terrorism". He noted that the officials had not provided any reason for staying away.

Justice Department spokesman Marc Raimondi said in an emailed statement that the department is "focused on aggressively and successfully pursuing terrorist adversaries" and on denying them "unchallenged recruiting platforms to spread their messages of hate and intolerance."

Democratic members of the subcommittee said national security policies more targeted at Muslims, such as Trump's call for a ban on Muslim immigrants, would be unconstitutional and counter-productive.

"We cannot fight radicalisation with further radicalisation," said Democratic Senator Christopher Coons of Delaware, adding at the end of the hearing that he did not think "arguing over semantics" for three hours had helped improve American national security.

Experts from Muslim and Arab advocacy groups as well as former government national security officials testified at the hearing, representing both sides of the debate over whether the administration emphasises Islam too much or too little in its counter-extremism policies.

Protesters with Code Pink, an anti-war advocacy group, offered Cruz packs of gum labelled "Islamophobin" as he entered the hearing room, telling him it could cure Islamophobia.

Additional reporting by Reuters.