VIENNA (AP) — An alliance of many of the world's biggest oil-producing nations extended their agreement to cut output for an additional nine months — an effort to support prices that will prove difficult in the face of growing production from the U.S.

Thursday's decision by the OPEC cartel — now at 14 members with the entry of Equatorial Guinea — and 10 other countries led by Russia, means that the reductions of 1.8 million barrels a day agreed on in November will stay in place until March.

Saudi Oil Minister Khalid A. Al-Falih, who presided over the meeting, said he expected that the extensio...