Crude oil output from the Organization of the Petroleum Exporting Countries climbed 320,000 barrels per day (b/d) to 31.71 million b/d in April from 31.39 million b/d in March, a just-released Platts survey of OPEC and oil industry officials and analysts showed.

"This is one of the more significant month-on-month changes Platts has reported in quite some time," said John Kingston, Platts global director of news. "First, it shows that sanctions clearly are taking a bite out of Iranian output. Also, this enormous one-month jump in production shows that, for now, OPEC has output capacity to cover the missing Iranian oil barrels. In the next few months Iranian output and overall OPEC production will be the key numbers to watch."

Production increases totalling 510,000 b/d from Angola, Iraq, Kuwait, Libya, Nigeria, Saudi Arabia and the United Arab Emirates(UAE) were partly offset by decreases totalling 190,000 b/d from Algeria, Iran and Qatar.

The drop in Iranian volumes to 3.28 million b/d from 3.4 million b/d the previous month comes as oil markets prepare for the imposition of U.S. financial sanctions on June 28 and a European Union ban on imports of Iranian oil on July 1.

Iran's exports to Europe have already dipped, partly due to banking restrictions in place that make it difficult for some refiners to process payments to the National Iranian Oil Company.

At the same time, the United States has been offering exemptions from the financial sanctions to countries which agree to significant reductions in their imports of Iranian oil. Japan has already been granted a waiver, having reached an agreement with Washington in March under which Japanese refiners will reduce their intake of Iranian oil by between 15% and 20%. Iran's other major oil customers in Asia are China, India and South Korea.

Iraq and Nigeria accounted for the two biggest increases – of 150,000 b/d and 100,000 b/d respectively – as the former boosted its exports to the highest level since 1979, before the start of the Iran-Iraq war, and the latter saw a new crude oil grade –Usan –join the export program.

According to Platts survey participants, output from OPEC kingpin Saudi Arabia was 9.95 million b/d, up 50,000 b/d from March but below the 10 million b/d reported earlier this week by Saudi oil minister Ali Naimi as current production.

April's total production puts OPEC output in excess of the group's 30-million-b/d ceiling, agreed in December 2011. The ceiling, which does not include individual country quotas, covers all 12 members including Iraq, which has been outside the group's quota system since it invaded neighbouring Kuwait in August 1990.

For production numbers by country, click here.