WASHINGTON -- UN Secretary General Ban Ki-moon issued a last-minute appeal to the United States Tuesday to halt the scheduled execution of a Mexican national sentenced to death for murder in defiance of an International Court of Justice (ICJ) order.

Jose Ernesto Medellin, 33, was due to be executed by lethal injection at 2300 GMT in the state of Texas. He was convicted of the 1993 rape and murder of two teenagers and is one 51 Mexican nationals on death row in the United States.

Less than 12 hours before Medellin's planned execution, Ban called on the US government to abide by an ICJ ruling calling for the executions of all Mexican nationals to be suspended.

"All decisions and orders of the International Court of Justice must be respected by states," Ban warned on local television in Mexico City where he was attending a world AIDS conference.

"The United States should take every step to make sure the execution does not take place," he added, saying he had taken all the necessary steps to delay Medellin's execution, detailed in a letter to the United States and that he was "confident" his demands would be accepted.

Mexico has complained that its nationals on death row were not informed of their right to consular access and assistance during trial, a right under the Vienna Convention.

Medellin's family anxiously waited at the prison near Houston where the 33-year-old is due to be executed at 2300 GMT by lethal injection.

Medellin's request for a reprieve was unanimously turned down on Monday by the Texas Board of Pardons and Paroles and his fate lies in the hands of Texas Governor Rick Perry.

Medellin's lawyers are still hoping the US Supreme Court will issue a stay of execution that would give the US Congress time to pass a new law that can force individual states such as Texas to abide by ICJ decisions.

The ICJ in 2004 ordered US officials to review the sentences for the Mexicans on death row and, on July 16 this year, instructed the US authorities to do everything then could to stay the imminent execution of five Mexicans, including Medellin.

The US House of Representatives took up such a bill after the ICJ's July 16 ruling.

Since the 2004 ruling, some US states have agreed to review their death row cases at President George W. Bush's request.

But Texas has refused, arguing -- with the support of a March US Supreme Court ruling -- that its state courts, which decided the Medellin case, are not bound by the ICJ treaty.

That left the federal government with no legal tools to force Texas to put off the execution.

Medellin's lawyers are also seeking at least an eight-month reprieve so that the Texas legislature -- in recess until January -- can approve a law allowing state or federal court hearings for their client and other Mexicans on death row.

"Should Texas execute Mr Medellin before Congress has a reasonable opportunity to convert the (ICJ) judgment into a justifiable federal right, the State of Texas will forever deprive Mr Medellin of his constitutionally protected right not to be deprived of his life without a due process of law," his lawyers told the Supreme Court in their motion this week.

Congress however is in recess until September. Any such measure would have to be approved by both the House and the Senate, then signed into law by the president.

Human Rights Watch, in a weekend statement, said that "executing Jose Medellin in violation of an order by the World Court would be a major step backward for the rule of law."

The Texas board on Monday also rejected a request by Medellin's lawyers' to commute his sentence to life in prison.

Medellin was sentenced to die for the rape and murder of two teenage girls -- aged 14 and 16 respectively -- in 1993 in Houston. The girls happened upon Medellin, who was 19 at the time, as he was engaged in a street gang initiation rite.