NSYNC star Lance Bass has vented his frustration at not being able to donate blood to Las Vegas massacre victims because he is gay.
Gay men cannot donate blood in the US if they have had sex with another man in the past year, reports The Sun.
Bass tweeted: "How is it STILL illegal for gays to donate blood??! I want to donate and I'm not allowed."
It came after Las Vegas mayor Carolyn Goodman made an appeal for blood donations following the horrific music festival shooting that left 59 dead and more than 500 others wounded.
Singer Bass, who announced he is gay in 2006, had earlier posted an emotional message saying he was praying for all those affected.
"Finding words to convey the depth of horror we are all witnessing in Las Vegas is just impossible," he wrote.
"It is unimaginable that another mass shooting of even greater scope than that of Pulse Nightclub could occur again in this country, but indeed, it has.
"We must work harder to stop these crimes that destroy human life.
"We pray for those whose lives were taken, as well as for the wounded and the hundreds who will forever be affected by this monumental tragedy."
The American Red Cross follows Food and Drug Administration (FDA) guidelines, which state: "Defer for 12 months from the most recent sexual contact, a man who has had sex with another man during the past 12 months."
"All US blood collection organisations must follow this federal requirement," the American Red Cross explains.
In the UK gay men are only allowed to donate blood if they have not had sex with another man in the past 12 months.
But the rules are set to change in early 2018, with gay men being able to donate blood in England three months after having sex.
This change has been made because of medical advances.
Fears over infections led to an outright ban on gay men giving blood at the height of the Aids epidemic.
Gunman Stephen Paddock opened fire on 22,000 people enjoying the Route 91 Harvest music festival in Las Vegas on Sunday.
Survivors of the massacre have told how the number of casualties could have been much higher had it not been for the brave few who risked their lives to tend to the wounded.
One of those hailed as a hero is dad-of-three Jonathan Smith, 30, who was in the city celebrating his brother's 43rd birthday when the attack took place.
It is believed Smith, a copy machine repairman from California, helped 30 people get out of harm's way before getting shot in the neck as hundreds of bullets hailed down from a hotel balcony.
He suffered a fractured collarbone, a cracked rib and a bruised lung. The bullet that hit him remains lodged in his neck which doctors fear that moving it might cause more damage.
"I might have to live with this bullet for the rest of my life," Smith told the Washington Post.
Many have praised Smith for his heroic actions.
"I don't see myself that way," he said. "I would want someone to do the same for me. No one deserves to lose a life coming to a country festival."
Stephen Paddock, 64, who had no criminal record, became America's deadliest gunman after firing several machineguns into a crowd in Las Vegas on Sunday night.
Killer Paddock was a millionaire retired accountant who flew small planes and had no criminal record, authorities said.
Many of the thousands of festival-goers attending the Route 91 Harvest festival did not realise at first that they were under attack as shots rained down from the 32nd floor of the Mandalay Bay Resort - with some thinking the noise was from fireworks.