Warren Buffett’s Berkshire Hathaway raised its stake in Occidental to about 23.6% after adding nearly 3.7 million shares.

The acquisitions, which Berkshire disclosed in a filing with the US Securities and Exchange Commission late Monday, cost about $216 million and took place between March 23 and 27.

Buffett’s company began buying heavily into Occidental shares more than a year ago when Russia invaded Ukraine and has spent more than $1 billion on the stock this month.

It now owns about 211.7 million Occidental shares, worth $12.6 billion, based on the oil company’s closing price of $59.65 on Monday.

In August, Berkshire received permission from the Federal Energy Regulatory Commission to buy 50% of Occidental’s common stock.

It also has $10 billion in Occidental preferred stock with an 8% dividend and warrants to buy another $5 billion in common stock at $59.62 each.

Berkshire is Occidental’s largest shareholder, and some analysts and investors have speculated that it could eventually buy the Houston-based company.

Buffett, 92, has been eager to make another big acquisition for his Omaha, Nebraska, conglomerate, whose dozens of businesses include Geico auto insurance and BNSF railroads.

Berkshire took a 22.6% stake in BNSF, then paid $26.5 billion for the rest in 2010.