Iraqi leaders have been unable to form a new government since the secular Iraqiya slate scored a narrow victory in March 7 elections for the Iraqi Council of Representatives. None of the leading slates won the majority needed to form a new government alone, and leaders have been at odds over candidates for president, speaker of Parliament and prime minister since.
U.S. officials in July tried to intervene by proposing a power-sharing agreement between top political leaders to no avail.
Relatives of the reclusive cleric said Obama in the letter called on Sistani to push Shiite leaders in Iraq to agree to settle their differences, the news magazine Foreign Policy reported Friday citing anonymous sources.
"It was a request for his intervention in the political situation to use his influence with the Shiite groups and get them to compromise," the source said.
Sistani follows the quietest sect of the Shiite faith, which advocates a strict separation between religious and political matters. His influence in Iraq, however, is immense.
Mike Hammer, a White House spokesman, told Foreign Policy there was no official comment.
The revelation comes as former Iraqi Deputy Prime Minister Tariq Aziz tells London's Guardian newspaper that U.S. forces were "leaving Iraq to the wolves" by adhering to a timeline to leave the country.
U.S. combat forces are scheduled to leave Iraq by the end of the month under the terms of a bilateral security agreement with Baghdad.
Aziz is in Iraqi custody for crimes against humanity.