Oklahoma's Supreme Court has ruled that a proposed "personhood" measure is "clearly unconstitutional," halting efforts in the state to grant rights to human embryos.

The high court decided unanimously Monday against the amendment that would define a woman's fertilized egg as a person.

Abortion opponents organized the group Personhood Oklahoma to get enough petition signatures for a November ballot vote. But the court determined the petition "is void on its face" and struck it down.

"The only course available to this court is to follow what the United States Supreme Court, the final arbiter of the United States Constitution, has decreed," the court said.

"We will consider our legal options in response to this," Personhood Oklahoma member Dan Skerbitz said. "We'll also have to reconsider our on-the-ground strategy here."

Opponents contend the personhood measure would ban all abortions and interfere with certain forms of contraception and procedures like in-vitro fertilization.

The ruling is the latest setback for pro-life supporters who have been pursuing personhood measures in several states.