"Secretary Clinton sought clarification on the Chinese government's policy, on the export of rare earth minerals and received assurances," a US official told reporters on condition of anonymity, after Clinton's meeting with Yang.
The meeting took place in Hanoi, on the sidelines of the 16-nation East Asia Summit, which the United States is attending for the first time, along with Russia.
In Washington, White House spokesman Robert Gibbs said Tuesday that the United States was checking to see whether China was cutting off rare earths exports to US companies but had not reached a conclusion yet.
China recently denied a report in The New York Times that it had halted some rare earth shipments to the United States in response to a US probe into alleged Chinese subsidies for its green technology sector.
Rare earths, a group of 17 elements, are used in high-tech products, ranging from flat-screen televisions to lasers and hybrid cars, and China controls more than 95 percent, of the global market.
China has cut rare earth exports by five to 10 percent a year since 2006 as demand and prices soar.
During talks with Japanese Foreign Minister Seiji Maehara in Hawaii on Wednesday, Clinton welcomed remarks from Beijing that China will not use its near-global monopoly on the rare earths trade as a "bargaining tool."
However, she said it was important to diversify sources and not rely solely on China.