Qualcomm Inc., the world’s biggest supplier of smartphone chips, suffered its worst stock decline in a month after upheaval in China threatened the company’s sales in a critical market.

The risks for Qualcomm are coming on multiple fronts. Apple Inc.’s iPhone is facing a widening ban in China that would bar the device from government-backed agencies and state companies. Qualcomm is one of the iPhone’s biggest vendors, responsible for the product’s modem chip.

And US lawmakers are scrutinizing suppliers to Huawei Technologies Co., with some calling for a halt to all US exports to the controversial Chinese tech company. San Diego-based Qualcomm provides Huawei with older-generation 4G chips.

China, including Hong Kong, accounts for roughly two-thirds of Qualcomm’s revenue. So investors are closely watching developments in that market. The stock plunged 7.2% to $106.40 Thursday on the latest news, alongside shares of other mobile-phone suppliers.

A representative for Qualcomm declined to comment.

The Huawei controversy centers on China’s top chipmaker, Semiconductor Manufacturing International Corp., which appears to have violated US sanctions by supplying advanced components to the blacklisted company, US lawmakers said.

“It sure looks like it did” violate sanctions, Representative Michael McCaul said Wednesday at a briefing at the US embassy in the Hague. SMIC continues “to try to get our intellectual property.”

Huawei uses an advanced 7-nanometer processor built by SMIC to power its latest smartphone, a teardown commissioned by Bloomberg News showed, indicating that China is making headway in its push to produce cutting-edge semiconductors despite US sanctions. It didn’t include a Qualcomm component, however.

Existing rules require any company that intends to supply Huawei with US technology, which is present throughout SMIC’s operations, to get approval from Washington. It’s unclear whether SMIC has a US license to supply Huawei.

McCaul, Republican chairman of the House Foreign Affairs Committee, has been vocal in protesting what he considers insufficient enforcement of the Entity List, which is enforced by the Commerce Department’s Bureau of Industry and Security and specifies license requirements imposed on restricted exports. Neither McCaul nor his committee have much direct sway over sanctions.

Representative Mike Gallagher, chairman of the House Select Committee on Competition with China, picked up on the theme, suggesting the US should end all its exports to both Huawei and SMIC — even those involving older technologies that are currently allowed under the law.

“The time has come to end all U.S. technology exports to both Huawei and SMIC to make clear any firm that flouts US law and undermines our national security will be cut off from our technology,” Gallagher said in a statement.

--With assistance from Cagan Koc, Sarah Jacob, Debby Wu and Ian King.

(Updates shares starting in fourth paragraph.)