An Iranian dhow seized off the Yemeni coast was carrying sophisticated Chinese antiaircraft missiles, a development that could signal an escalation of Iran’s support to its Middle Eastern proxies, alarming other countries in the region and renewing a diplomatic challenge to the United States.

Among the items aboard the dhow, according to a review of factory markings on weapons and their packing crates, were 10 Chinese heat-seeking antiaircraft missiles, most of them manufactured in 2005.

The missiles were labeled QW-1M and bore stencils suggesting that they had been assembled at a factory represented by the state-owned China National Precision Machinery Import and Export Corporation, sanctioned by the United States for transfers of missile technology to Pakistan and Iran.

The Chinese missiles were part of a larger shipment interdicted by American and Yemeni forces in January, which American and Yemeni officials say was intended for the Houthi rebels in northwestern Yemen. But the presence of the missiles in the seized contraband complicates an already politically delicate case.