OPEC crude oil output rose in August as Iranian shipments climbed slightly from its lowest in more than two decades and because of higher exports from Angola and Nigeria, a Reuters survey showed on Thursday.

Supply from the 12-member Organization of the Petroleum Exporting Countries averaged 31.53 million barrels per day (bpd), up from 31.30 million bpd in July, the survey of sources at oil companies, OPEC officials and analysts found.

The most notable trend in August is the lack of a further decline in Iranian exports, which have dropped sharply this year due to U.S. and European sanctions. Supply rose by 50,000 bpd in August, according to the survey, to 2.85 million bpd.

Iran's crude became subject to a European Union embargo from July 1. The embargo also bars EU insurance firms from covering Iran's exports, hindering imports by non-EU buyers.

More crude is heading in August to some Asian customers such as Japan, where government-backed shipping insurance has encouraged purchases despite the EU sanctions, sources in the survey said.

However, Iran's supply remains near a historic low. July's output was its lowest since 1988, when it pumped 2.24 million bpd, according to figures from the U.S. Energy Information Administration.

Angolan and Nigerian exports, which both tend to be volatile on a month-by-month basis, climbed in August, pushing total OPEC production higher.

Angolan shipments reached a 2 1/2-year high due to extra cargoes of some crude grades and completion of maintenance at the Girassol oilfield, one of its larger production facilities.