A memorial for LGBT veterans in Abraham Lincoln National Cemetery has been defaced with a tar-like substance.

Unveiled in 2015, the monument in the Abraham Lincoln National Cemetery, Illinois, is the first federally-approved monument honoring LGBT veterans anywhere in the United States.

The monument pays tribute to the brave LGBT servicepeople who have suffered and lost their lives.

In a shocking act of vandalism, the memorial was defaced this week.

According to local media, a thick black substance like roof tar was poured over the monument, obscuring a large pink triangle and much of the text.

The crime has stunned the authorities who maintain the cemetery, which is overseen by the Department of Veterans Affairs.

Cemetery director Sean Baumgartner told CBS: “My reaction, it was horrible. Who would do that?

“Who would take the time to deface a monument on such a sacred ground. It’s ridiculous.

“It was really a slap in the face to veterans everywhere and it was really uncalled for.

“I’d love to see them come forward and own up to what they did, you know take their medicine.”

The vandalism was discovered last Friday, June 16.

Workers quickly set to work cleaning the monument, which has now been restored with no permanent damage.

Police are investigating the crime – and if found, the vandals could face serious charges.

Chief Fred Hayes, Elwood Police Department told ABC: “Any kind of damage to federal property is an enhanced penalty.

“So that person would face state charges and possibly federal charges for damaging a national monument.”

The caption on the memorial reads: “Gay, lesbian, bisexual and transgender people have served honourably and admirably in America’s Armed Forces.

“In their memory and in appreciation of their selfless service and sacrifice, this monument was dedicated.”

The dedication of the monument in 2015 was a major step, just a few years after the repeal of ‘Don’t Ask, Don’t Tell’, which had forced LGB troops to stay in the closet.