Image copyright Getty Images Image caption Oklahoma Governor Mary Fallin said the proposed bill was vague and ambiguous

Oklahoma Governor Mary Fallin has vetoed a bill that would make abortion a criminal offence in the US state.

Although she opposes abortion, she said the measure was vague and would not withstand a legal challenge.

The state senate on Thursday backed the bill that would have punished doctors who terminate a pregnancy with up to three years in prison. They would also be barred from practising medicine.

To override the veto, lawmakers require a two-thirds majority in each chamber.

Abortion is legal in the US, and abortion rights activists have already described the bill as unconstitutional.

Two abortion clinics remain open in Oklahoma after the state recently enacted a number of new regulations affecting the facilities.

Ms Fallin, a rising star in conservative circles, has been mentioned as a potential running mate for Republican presidential candidate Donald Trump.

Image copyright Getty Images Image caption Activists hope to overturn Roe v Wade, the legal case that made abortion legal

Anti-abortion groups said they hoped to use the bill to trigger a legal case that would overturn Roe v Wade, the Supreme Court verdict that made abortion legal in 1973.

Legislation similar to that in Oklahoma was enacted in Utah and Louisiana in 1991 - but the laws were later ruled unconstitutional.

In recent years, conservatives have sought to tighten restrictions on abortion clinics and doctors rather than seek an outright ban.

Abortion rights advocates say these measures are meant to restrict women's access to abortion.

The new laws are particularly widespread in conservative southern states.

This week, South Carolina's legislators passed a bill that would bar abortions after the 19 weeks of pregnancy.