A year ago CEFC agreed to pay $680 million for 51 percent of KMG International, a fully-owned unit of Kazakh state oil and gas firm KazMunayGaz that owns a key refinery in Romania and nearly 1,000 petrol stations in Europe.

But conclusion of that deal got held up after the Romanian government seized the refinery and other assets belonging to Rompetrol and parent company KazMunayGaz in a probe of the Romanian company's privatisation in 2000.

Under a new deal agreed last month in Romania, CEFC took over 51 percent of KMGI paying "a small portion" of the $680 million, and will pay the remainder once the probe is over, said two CEFC officials with direct knowledge of the matter. The officials declined to specify the size of the payment.

A CEFC press official confirmed the agreement of the stake transfer, but did not elaborate on the details. KMGI officials were not immediately available for comment.

"The additional agreement, which sets new clauses, allows CEFC and KMGI to move on with executing some of our future investment plans," said one of the CEFC executives.

"With this agreement, CEFC is now legally the majority holder of KMGI," said the second executive.

CEFC officials have said the company plans to acquire more service stations and also invest in storage facilities and terminals in the Black Sea region, using KMGI's European assets as a platform.

Among KMGI's key assets are its 100,000 barrels-per-day Petromidia refinery in the Black Sea port of Constanta and a 400,000 tonnes-per-year fertiliser plant in southern Romania, along with nearly 1,000 petrol stations in Romania, Spain, Georgia and Bulgaria.

One of the CEFC executives said the $680 million transaction may take two years to complete depending on the speed of regulatory approvals in countries where Rompetrol holds assets.

(By Reuters; Reporting by Chen Aizhu; Editing by Tom Hogue)