It was not immediately clear why Rosneft changed its allocation of ESPO crude, a popular grade among Chinese "teapots", or independent refineries that collectively make up a fifth of the country's crude imports.

But the changes may suggest that Rosneft's deal with CEFC, which was supposed to end in 2022, might have been adjusted or even terminated earlier than expected, the sources told Reuters.

Rosneft did not respond to a request for comment. Calls to CEFC in Singapore went unanswered. Trafigura declined to comment.

"There was a discussion going on for a while to cut supplies to CEFC ... it looks like Trafigura could get it," said one of the sources familiar with the talks.

A change to Rosneft's crude supply deal would be another blow to CEFC, which has been under pressure since its founder and chairman Ye Jianming was detained by Chinese authorities in 2018.

Trafigura [TRAFGF.UL] received six cargoes for April loading and the Swiss commodities trading house could become the major lifter of Rosneft's ESPO crude this year, replacing CEFC, two of the sources said.

Trafigura, which was the term lifter of Rosneft's ESPO before CEFC took over in 2018, has since sold one of the April cargoes to oil major Royal Dutch Shell, traders said.

Rosneft had pledged to supply CEFC with 61 million tonnes of oil over five years starting from Jan. 1, 2018. In 2019 the Russian firm switched its crude supply deal to CEFC's trading unit in Singapore from the China-based unit.

Rosneft also supplies Urals crude to CEFC under the contract and it is unclear if this supply will be affected from April, traders said, adding that Rosneft has allocated at least one 80,000-tonne Urals cargo to CEFC to load from Novorossiisk in March.

By Olga Yagova and Shu Zhang