(CNN) -- A roadside bomb that struck a U.S. State Department convoy in Baghdad, killing an American contractor, is believed to be the work of Shiite militias who have stepped up attacks against foreigners in recent months, the U.S. military said Friday.
The attack comes as Iraq debates whether to request American soldiers stay beyond a January 1, 2012, deadline that requires 46,000 troops out of the country. Anti-American Shiite cleric Muqtada al-Sadr has vowed to "escalate armed resistance" with his Mehdi Army militia, if the U.S. military does not leave Iraq by the end of the year.
"We have not seen a claim of responsibility, but the target and tactics resemble the activities of one of the three main Iranian-backed militias in Iraq: Kataib Hezbollah, Asaib al Haq, and the Promise Day Brigade," Army Maj. Gen. Jeffrey Buchanan, spokesman for U.S. Forces-Iraq, told CNN Friday in an e-mail.
Iran has long denied the U.S. military charge that it backs the Shiite militias.
The blast Thursday that killed Stephen Everhart, who was working for the U.S. Agency for International Development, was the second in a week in the Iraqi capital that targeted embassy personnel. A French Embassy convoy was struck Monday by a roadside bomb in Baghdad. No embassy personnel were killed or injured in the attack.
It also follows a number of attacks this month against the U.S. military that have resulted in the deaths of nine soldiers, the largest loss of life among American troops this year in a single month.
While violence has fallen off dramatically since the height of violence in 2006 and 2007, there has been a noticeable increase in insurgent attacks in recent months.
"The security environment remains complex, but with our partners in the Iraqi security forces we are determined to maintain pressure on all groups attempting to destabilize Iraq and cause harm," Buchanan said.
Iraqi forces tightened security Friday around a revered Shiite holy site in Baghdad where thousands were expected over the weekend to make an annual pilgrimage to begin making an annual pilgrimage to the Khadamiyah Shrine that holds the tomb of a Shiite imam.
The move comes a day after three bombs exploded in rapid succession near a mosque and an outdoor market in a Shiite neighborhood in southern Baghdad, killing 21 people and wounding 117, said an Iraqi police official.
The official spoke on condition of anonymity because he was not authorized to speak to the media.