Two more women have died after using the abortion pill RU-486, federal health regulators said Friday, in warning doctors to watch for a rare but deadly infection implicated in earlier deaths.

At least seven U.S. women have died after taking the pill, sold since 2000. The Food and Drug Administration cannot prove the drug was to blame in any of the cases.

The FDA has not confirmed the cause of the latest two deaths.

However, in four of the earlier cases, the women died from an infection of the bloodstream, or sepsis. Those women did not follow FDA-approved instructions for the pill-triggered abortion, which requires swallowing three tablets of one drug, followed by two of another two days later.

Most abortion clinics instruct that the second course of pills be inserted in the vagina, as occurred in those four earlier deaths. Studies show that method works, but it is considered a so-called "off-label" use of the abortion pill.