JEFFERSON CITY -- The Missouri Supreme Court declined Tuesday to set execution dates for six inmates currently sitting on death row until a legal dispute over the state's execution method can be resolved.

Earlier this year Missouri stopped using its longstanding three-drug method for executions in favor of a single-dose drug called propofol.

Propofol has never previously been used as an execution drug, prompting concerns that it could result in pain and suffering for the condemned. A lawsuit currently pending in Cole County court contends the use of propofol violates the constitutional ban on cruel and unusual punishment.

Until the legal questions are answered, "ruling on the motion to set execution date is premature," the court ruled Tuesday.

Only two men have been executed in Missouri since 2005. Earlier this year, Attorney General Chris Koster asked the state Supreme Court to set execution dates for 19 death row inmates. The request came shortly after the state changed its lethal injection protocol due to a national shortage of sodium thiopental, the first part of the three-drug execution method.

In a statement, Koster said he was disappointed with the Supreme Court’s decision. The cases involved have been completely through the appeals system and are ready to have execution dates set, he said.

“The Court had the option of setting execution dates, which would have effectively imposed deadlines on lower court challenges to the execution protocol,” Koster said. “The Attorney General’s office will continue to do all we can to expedite the protocol-challenge cases.”

Among the six cases included in Tuesday's ruling was Allen Nicklasson, of Kansas City, who was sentenced to death for the 1994 murder of Richard Drummond, a good Samaritan who had stopped to help him while stranded on Interstate 70. An accomplice in that murder, Dennis Skillicorn, was put to death in 2009.