An Iranian boat seized off the Yemeni coast was carrying sophisticated Chinese anti-aircraft 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 US.

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

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

The analysis of the weapons' markings and origins was based on photographs taken when Yemeni officials briefly displayed the weapons to journalists.

In 2008, late in the Bush administration, the US complained to China about two similar anti-aircraft missiles that were recovered from Shiite militants in Iraq, according to a diplomatic cable made public by WikiLeaks.

''We have demarched China repeatedly on its conventional arms transfers to Iran, urging Beijing to stop,'' a cable noted.

The Chinese missiles were part of a larger shipment interdicted by US and Yemeni forces in January, which US and Yemeni officials say was intended for Shiite Houthi rebels in north-western Yemen.

The shipment, which officials portray as an attempt to introduce sophisticated new anti-aircraft systems into the Arabian Peninsula, has raised concerns in Saudi Arabia, Oman and Yemen, as the weapons would have posed escalated risks to civilian and military aircraft alike.

And it has presented the Obama administration with a fresh example of Iran's apparent transfer of modern missiles from China to insurgents in the larger regional contest between Sunni-led and Shiite-led states, in which the US military has often been entwined.

The US has previously accused Iran of sending weapons to the Houthis, who follow an offshoot of Shiite Islam. Saudi Arabia, a US ally, is considered the leading Sunni power in the region.

Both sides have aided and equipped groups or governments they deem aligned with their interests, helping to fuel violence in Yemen, Iraq, Syria, Lebanon, the Palestinian territories, Sudan and elsewhere.

Iran has rejected the allegations as ''baseless and absurd''. Neither the Iranian government nor the Chinese company that markets QW missiles answered written requests for comment.

The government of Yemen has asked the United Nations to investigate the shipment and report the findings to the Security Council. Yemeni news media reported that UN experts were in Yemen last week.

The New York Times