A 53-year-old white man convicted of two racially-motivated murders in 1987 has been executed in Florida by a lethal injection that included a drug never before used in a US execution, state officials said.

Authorities said Mark Asay was pronounced dead at 6:22pm local time (22:22 GMT) on Thursday.

He was the first white man to be executed in Florida for killing a black victim, according to the Death Penalty Information Center.

Prosecutors said Asay made racist comments in the 1987 fatal shooting of Robert Lee Booker, a 34-year-old black man.

Asay also was convicted of the 1987 murder of 26-year-old Robert McDowell, who was mixed race, white and Hispanic.

READ MORE: The struggle against America's racist death row

When asked whether he wanted to make a final statement, Asay said, "No sir, I do not. Thank you".

The execution protocol began at 6:10pm local time (22:10 GMT). Within minutes he was motionless and was subsequently pronounced dead by a doctor.

At least 20 black men have been executed for killing white victims since the state reinstated the death penalty in 1976, according to data from the Death Penalty Information Center. A total of 92 Florida inmates had been executed previously in that time period.

Controversial drug

Asay received a three-drug injection that began with the anaesthetic, etomidate.

Etomidate is the first of three drugs administered in Florida's new execution mixture. It is replacing midazolam, which has been harder to acquire after many drug companies began refusing to provide it for executions.

The etomidate is followed by rocuronium bromide, a paralytic, and finally, potassium acetate, which stops the heart.

While the state's high court has approved the use of etomidate, some experts have criticised the drug as being unproven.

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State corrections officials have defended the choice, saying it has been reviewed.

Doctors hired by Asay's attorneys raised questions about etomidate in court declarations, saying there are cases where it had caused pain along with involuntary writhing in patients.

But in its opinion allowing the drug to be used, the state's high court said earlier this month that four expert witnesses demonstrated that Asay "is at small risk of mild to moderate pain".

Executions in Florida were put on hold for 18 months after the Supreme Court ruled that the old system was unconstitutional because it gave judges, not juries, the power to decide.

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Since then, Florida's legislature passed a law requiring a unanimous jury for death penalty recommendations.

In Asay's case, jurors recommended death for both murder counts by a 9-3 vote. Even though the new law requires unanimity, Florida's high court ruled that the US Supreme Court's ruling did not apply to older cases.

Asay was the 24th inmate executed since Gov. Rick Scott has taken office, the most under any governor in Florida history.