LONDON (Reuters) - OPEC oil supply fell in April to its lowest this year as a strike cut Nigerian output and top OPEC exporters Saudi Arabia and Iran trimmed production, a Reuters survey showed on Thursday.

Output from the Organization of the Petroleum Exporting Countries slipped to 31.64 million barrels per day in April from 32.05 million bpd in March, according to the survey of oil firms, OPEC officials and analysts.

The 12 members bound by output targets, all except Iraq, pumped 29.41 million bpd, down from 29.75 million bpd in March and below their target of 29.67 million bpd, the survey found.

OPEC pumps about two in every five barrels of oil.