The deal will mark a retreat from the world's third-largest economy by the US oil giant, which is focusing its resources globally on exploration and production and shifting away from so-called downstream assets, such as its sales network in Japan. 

The move could also spark another round of consolidation in an industry reeling from sluggish demand due to Japan's sputtering economy and increased use of more environmentally-friendly energy sources. 

"If this report is true, it could open the door for TonenGeneral to realign in alliance with another company. That may or may not include capital ties," said Hidetoshi Shioda, senior analyst at SMBC Nikko Securities. 

Shares of TonenGeneral, owned 50.02 percent by Exxon, last traded down 5.5 percent at 795 yen - a near 14-month low and set to be the biggest one-day fall in almost 10 months - reflecting investor worries over how it would fund the share purchase. Trading in the shares was the busiest in more than 22 years.  

The entire stake is worth 228 billion yen ($3 billion) at the current market price. 

TonenGeneral imports and distributes oil from the Exxon group in Japan, where it ranks second in oil refining capacity after JX Holdings, the industry leader created in a merger of refiners and miners in April 2010. 

Other rivals include Idemitsu Kosan Co., Cosmo Oil and Showa Shell. 

In addition to its stake in TonenGeneral, Exxon is also in talks to sell its sales operations, which include a nationwide network of about 4,000 gas stations, to its Japanese partner.  

In total, a deal could top 400 billion yen ($5.2 billion), according to the sources, who spoke on condition of anonymity due to the sensitivity of the matter. 

While holding on to a reduced stake, Exxon plans to keep supplying oil to TonenGeneral, the sources said, adding that an official announcement could come as early as this month with an aim to finalizing the deal in the spring. 

"Exxon does not comment on speculation. ExxonMobil has no plans to exit the Japan market," the oil giant said in a statement to Reuters. 

TonenGeneral plans to borrow money from several banks to finance the purchase of its shares from Exxon, the sources said. 

Total oil demand in Japan, the world's No.3 consumer, has been on a steady decline, dropping to about 3.4 million barrels per day (bpd), from a record 4.2 million bpd in 1999. 

That slide has accelerated following a devastating earthquake and tsunami in March that dented economic activity.