GENEVA — An Iraqi lawyer known for her work promoting women’s rights has been killed by Islamic State fighters, the head of the United Nations human rights office said on Thursday, continuing a pattern of attacks on professional women.

The lawyer, Sameera Salih Ali al-Nuaimy, was seized from her home by Islamic State fighters last week and tortured for several days before a masked firing squad executed her in public on Monday, Zeid Ra’ad al-Hussein, the United Nations human rights commissioner, said in a statement.

Ms. Nuaimy had posted comments on her Facebook page condemning the “barbaric” bombing and destroying of mosques and shrines in Mosul, a northern Iraqi city, by the Islamic State, the militant group also known as ISIS or ISIL. She was convicted of apostasy by a “so-called court,” Mr. Zeid said, adding that her family had been barred from giving her a funeral.