BEIRUT, Lebanon - The United States has suspended delivery of nonlethal aid to the armed opposition in northern Syria after reports that Islamist militias had seized U.S.-provided equipment in the country, the American Embassy in Turkey confirmed Wednesday.

The move appears to be the latest blow to the Western-backed Free Syrian Army, which has been increasingly overshadowed on the battlefield by hard-line Islamist insurgents, including several groups linked to al-Qaeda.

Meanwhile, Turkish authorities said they had closed the major Bab Hawa border crossing to Syria after the seizure by Islamist rebels of the gate on the Syrian side.

The White House strategy has been based on supporting so-called moderate elements in the opposition fighting to oust the government of President Bashar al-Assad. However, reports from Syria indicate that al-Qaeda affiliates and other hard-line Islamist militias - some supported by Saudi Arabia, a close U.S. ally - are emerging as the most powerful factions in the deeply fractured opposition ranks.

The ascendancy of hard-line Sunni Islamist militias has caused deep concern in Washington about the direction of U.S. policy in Syria, which is engulfed in the third year of a withering conflict that has heightened instability and tension throughout the Middle East. Some rebel militias have an openly sectarian and nondemocratic agenda for the nation's future.

T.J. Grubisha, a spokesman for the U.S. Embassy in Ankara, Turkey, confirmed the suspension of nonlethal aid Wednesday in a telephone interview. But he emphasized that the move did not affect shipments into northern Syria of humanitarian aid, which is distributed via international relief organizations.

British officials also moved to suspend aid to northern rebels.

Turkey, which shares a more-than-500-mile border with Syria, is a key conduit for all manner of aid and equipment being shipped into rebel-held zones in northern Syria. Aid groups also have expressed concern about armed groups hijacking aid trucks and equipment in Syria and kidnapping drivers and aid workers.

The United States has provided more than $115 million in so-called nonlethal support to the Syrian opposition. Though the embassy spokesman would not provide details on the type of aid suspended, the nonlethal supplies include handheld two-way radios and other communications equipment, medical kits, laptop computers, trucks, and satellite-access gear.

Prompting the suspension were reports last week that armed Islamist opposition fighters, possibly linked to al-Qaeda, had seized opposition supply depots on the Syrian side of the border. The seizure appeared to be yet another indication of the relative weakness of the Free Syrian Army, which has long been a loose umbrella franchise of sundry rebel militias that lacks a central command structure.

U.S. officials have previously expressed concern that equipment meant for the Western-backed forces could fall into the hands of al-Qaeda militias or other extremist factions. It was not yet confirmed, however, if any U.S.-provided stock had been seized.

Grubisha said U.S. authorities were in contact with Gen. Salim Idriss, the nominal leader of the Free Syrian Army, to determine the status of goods destined for his group.

State Department officials said they had no information indicating that weapons or other lethal equipment belonging to the Free Syrian Army were seized. "Our understanding at this point was that it was nonlethal," State Department spokeswoman Jen Psaki said.