Oklahoma's governor on Wednesday signed into law the United States' strictest abortion ban, prohibiting the procedure in the state from the moment of conception.

The law, which also allows criminal proceedings to be brought against anyone who assists a person to get an abortion, came into force with Kevin Stitt's signing of the bill.

"I promised Oklahomans that as governor I would sign every piece of pro-life legislation that came across my desk and I am proud to keep that promise today," the Republican said in a statement.

"From the moment life begins at conception is when we have a responsibility as human beings to do everything we can to protect that baby's life and the life of the mother."