America's largest Islamic civil rights group is calling on Muslims to give blood in the aftermath of the most deadly mass shooting in US history.

The Council on American-Islamic Relations condemned the “monstrous” attack on the Pulse gay nightclub in Orlando, and offered condolences to the families of those killed or injured.

"We condemn this monstrous attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured," a statement by the group said.

"The Muslim community joins our fellow Americans in repudiating anyone or any group that would claim to justify or excuse such an appalling act of violence."

Michael Cheatham, a surgeon for Orlando health, told a news conference that hospitals in the area had implemented a “mass casualty plan” and urged donors to go to blood banks rather than to hospitals.

One Blood, which organises blood donation, said O negative, O positive and AB plasma donors were urgently lacking.

Here's the line at a local blood bank for Pulse Shooting victims. One man said, "This is the real Orlando." @bn9 pic.twitter.com/KmX8EU9Ld7 — Erin Maloney (@ErinOnTV) June 12, 2016

Meanwhile, the ban on gay men donating blood has been lifted, with One Blood announcing it was “in crisis and will be accepting all donors”.

It said all donors would be given full blood screening.

The organisation said it had been having difficulties with its website amid a spike in interest.