MOSCOW, (Reuters) – An oil row between Russia and China has escalated this week, industry sources said on Friday, as Moscow believes Beijing has underpaid it $100 million for oil in 2011 under a landmark 20-year supply deal.

Kommersant newspaper reported last week Russia’s state oil major Rosneft and China’s National Petroleum Corp disagreed over $38 million in payment for January.

Industry sources told Reuters on Friday Moscow believes the sum has now risen to $100 million for January and February.

CNPC was not available for comment. Russia’s pipeline monopoly Transneft said the two sides had some disagreements but declined specific comment while Rosneft said it was delivering supplies according to the contract.

“We have certain disagreements (with China), but we expect that we will sort them out,” said Transneft’s spokesman Igor Dyomin. “We assume that we have contracts and both sides should adhere to them. We have fully met our obligations so far.”