Iran is finding a way around Western sanctions to export increasing amounts of an ultralight oil to China and other Asian markets, expanding the value of its trade by potentially billions of dollars a year.

The exports come during a slight thaw in Iran’s relations with the West as negotiations over its nuclear program continue, and energy experts say it is counting on the United States and Europe to tolerate an increasing export stream.

According to Iranian customs data, the country in recent months has exported 525,000 barrels a day of the ultralight oil, known as condensates, over two times more than it did a year ago. In the last three months, the sales have generated as much as $1.5 billion in extra trade — a rate of about $6 billion a year — based on Iranian trade figures and market prices, analysts said.

The result has been an overall increase in petroleum shipments to China and other Asian markets without violating the letter of the sanctions. American officials have aimed to keep Iranian oil exports at around one million barrels a day, but when combined with condensate sales, they averaged 1.4 million barrels a day between January and May, according to the Energy Department.