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US lawyers are preparing to challenge Ohio's choice of death-penalty drugs after a murderer took 25 minutes to die from a new cocktail of chemicals.

Dennis McGuire, 53, suffered an "agonising" death in violation of his constitutional rights, lawyers said.

McGuire family lawyer Jon Paul Rion says he intends to launch a legal challenge against the new drug.

Witnesses said McGuire, who raped and killed a pregnant woman in 1989, gasped for at least 10 minutes before he died.

"He gasped deeply, there was kind of a rattling, guttural sound, there was kind of a snorting through his nose," said one unidentified witness.

"A couple of times he definitely appeared to be choking."

Ohio was forced to change its lethal injection to a new two-drug cocktail after the Danish maker of the previous execution drug refused to allow its use in capital punishment.

Mr Rion said the execution amounted to "cruel and unusual" punishment that violated the Eighth Amendment of the US constitution.

He said he would hold a news conference later to announce a legal challenge.

Analysts said it was one of the longest executions since Ohio resumed capital punishment in 1999.

McGuire's defence lawyer Allen Bohnert said his death was "a failed, agonising experiment".

"The people of the state of Ohio should be appalled at what was done here today in their names," he said.

In recent years, US states have struggled to get drugs for use in lethal injections.

Ohio officials chose to use the sedative midazolam and painkiller hydromorphone for McGuire's execution.

Federal judge Gregory Frost rejected appeals arguing that the drugs could cause excess suffering.

Berkeley University's Megan McCracken, who advises lawyers challenging lethal injection practices, told the ´óÏó´«Ã½ that "at the very least we can say this execution did not proceed as planned".

She said lethal drugs are not expected to act instantly, but McGuire's death "raised concerns".

"Under our constitution the question is whether the execution procedure presents a substantial risk of serious harm," she said.

"An execution that takes a long time certainly starts to implicate the eighth amendment."