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The US state of Texas is running out of a key drug used for lethal injections, according to officials.

Texas Department of Criminal Justice spokesman Jason Clark said that the state's supply of pentobarbital would end in September.

Texas has the highest execution rate in the country, with 11 inmates put to death so far in 2013.

Some drugs companies have objected to their products being used for capital punishment.

Texas has used pentobarbital, a drug used to treat severe epilepsy, in executions since July 2012.

The state was forced to change to the single-dose sedative when supplies of sodium thiopental, one of three drugs used previously, were cut off.

But state officials are now having difficulties with the availability of pentobarbital, which is also typically used to put down animals.

"We will be unable to use our current supply of pentobarbital after it expires,'' the Associated Press news agency quoted department spokesman Jason Clark as saying. "We are exploring all options at this time."

The state is planning at least five more executions this year.

Reuters quoted Mr Clark as saying that Texas was confident it would be able to continue with the deaths, despite the shortage.

Richard Dieter, who heads the Washington-based Death Penalty Information Center, said other states would be having similar problems.

"The states really scramble to go all over to get drugs,'' he told AP.

"Some went overseas, some got from each other. But these manufacturers - a number them are based in Europe - don't want to participate in our executions. So they've clamped down as much as they can."

The first US prisoner put to death using pentobarbital is believed to be 58-year-old John David Duty.

Duty, who killed a cellmate in 2001, died in Oklahoma in December 2010.

Texas is reported to have executed more than 500 death row inmates since 1982 - the highest of any US state. Virginia came in second with 110, AP said.