Authorities in the southern US state of Texas have executed an African American man despite protests from human rights groups that said he was mentally disabled.

Marvin Wilson, 54, who was condemned to death for a killing a police informant in 1992, was declared dead by lethal injection at 11:27pm GMT, according to a release from the Texas Department of Criminal Justice.

The US Supreme Court rejected a final appeal from defense lawyers in the hours before the execution.

Amnesty International called the decision "highly disturbing" and several other rights groups criticised the sentence.

In 2004, Wilson was diagnosed with mild mental retardation, with an IQ of 61 – well below the average for his age, according to the American Association of Intellectual and Developmental Disabilities (AAIDD).

"We are gravely disappointed and profoundly saddened that the US Supreme Court has refused to intervene to prevent tonight's scheduled execution," Wilson's attorney, Lee Kovarsky, said earlier Tuesday.

"It is outrageous that the state of Texas continues to utilize unscientific guidelines... to determine which citizens with intellectual disability are exempt from execution," he added.

Prior to his execution, Wilson told family members he loved them.

"Take me home Jesus. Take me home Lord. I love you all. I'm ready," Wilson said in his final statement, according to authorities.

In 2002, the Supreme Court ruled against executing mentally disabled convicts but left it to individual states to determine what constitutes a mental handicap.

According to the Texas definition, Wilson did not suffer from mental disability.

Last month, another African American diagnosed with a mental disorder, Yokamon Hearn, 34, was executed in Texas despite international protests.

Wilson is the 25th convict executed in the United States this year and the seventh in Texas.