Warren Buffett’s Berkshire Hathaway is ramping up buying in a secret stock that could be in the banking sector. For two straight quarters, the Omaha-based conglomerate has asked regulators not to disclose one or more of its purchases, suggesting to investors that Berkshire has been building a stake in a new position . If Berkshire’s latest earnings report is any indication, the new stock could be in the financial sector. In the fourth quarter, the conglomerate’s cost basis for “banks, insurance, and finance” equity holdings jumped by $2.376 billion. That’s larger than the $1.213 billion increase in the third quarter, indicating Berkshire accelerated buying in whatever position it’s building. Still, the bulk of Berkshire’s holdings was concentrated in just five companies in 2023. For the year ending December, the conglomerate noted the top five companies accounted for about 79% of the aggregate fair value, up from 75% in 2022. Apple , which is the Oracle of Omaha’s biggest holding, amounted to a $174.3 billion stake at the end of December, up $55.3 billion from $119 billion in 2022. Apple shares surged roughly 48% in 2023. However, it’s down this year by more than 5%. The conglomerate’s stake in Chevron , its fifth-largest holding, dropped considerably in 2023. In December, its stake in the oil giant amounted to $18.8 billion, down by roughly 37% from $30 billion in the prior year. Chevron shares were down by more than 16% in 2023. Berkshire’s American Express stake rose to $28.4 billion in December from $22.4 billion in the year-ago period. Bank of America was worth $34.8 billion, up from $34.2 billion. The Coca-Cola investment decreased to $23.6 billion from $25.4 billion. — CNBC’s Alex Crippen contributed to this report.