They said oil intake in Transneft's nationwide network, which handles about 85% of Russia's total crude output, was about 8.8 million barrels per day (bpd) in the 12-day period, citing data that included oil used at home and export volumes.

The sources did not say whether the drop was due to an oil contamination issue last month that disrupted flows through the Druzhba pipeline which supplies oil to refineries in East European and as far west as Germany.

At least 5 million tonnes of oil, or about 36.7 million barrels, was tainted with organic chloride, a compound used to boost oil extraction but which needs to removed before being pumped to refineries to avoid damaging processing equipment.

Transneft has proposed mixing contaminated oil with the clean crude at the Russian Black sea port of Novorossiisk, which was not initially affected by the issue, so that it can be sold, after some international traders refused to take tainted oil.

Industry sources said separately on Monday that levels of the organic chloride in oil loading from Novorossiisk have started to rise since early May but remained inline with allowed maximum norm of 10 parts per million (ppm).

Russian officials and Transneft had promised to fix the contamination issue by May 6, but the date has been pushed back.

Total Russian oil production has also slipped, declining to 11.16 million bpd from May 1 to May 12 from an average of 11.23 million bpd in April, the sources said.

This is the lowest output level since June, when it was 11.06 million bpd.

Russia has agreed with the Organization of the Petroleum Exporting Countries and other producers to lower its output in a bid to shore up global prices, which have partly been supported by the disruption to the Druzhba pipeline.

Its target level in the deal is 11.18 million bpd, so the data provided by the sources indicates Russian volumes at the start of May were lower than those required under the deal.

The Energy Ministry did not immediately respond to a Reuters request for comment.