U.S. officials said they believe the Iranians, detained in northern Iraq in January 2007, had facilitated attacks on American-led forces but handed them over to the Iraqi government at its request because they were obliged to do so under a U.S.-Iraqi security agreement.

The U.S. State Department said it was concerned their release could present a security threat to American troops in Iraq.

Iraq's foreign minister, Hoshyar Zebari, called the release a "good initiative" that could encourage dialogue between Washington and Tehran.

Iranian Embassy spokesman Amir Arshadi said Iraq had transferred the Iranians, described by their government as diplomats, to the embassy. Washington believes they are associated with the Quds Force, part of Iran's powerful Revolutionary Guard Corps, and that they trained Iraqi militants.

Attacks are down sharply from past years of war and militants have been driven from many strongholds, but they routinely inflict casualties in Baghdad and northern Iraq, a cauldron of ethnic and sectarian tension.

The most lethal attack yesterday was in the northern city of Tal Afar, where women sat in the street amid torn and bloodied bodies in the aftermath of suicide bombings, wailing and beating their chests in grief. Several men crouched and wept into their hands. Others rushed the wounded to ambulances, using bed sheets as makeshift stretchers. *