"Anbar operations command has been cleared," Muhannad Haimour, spokesman and adviser to the provincial governor, told AFP. Several security officials confirmed the retreat.
Asked about casualties in the ranks of government forces, Haimour said: "Many but we don't have an exact count."
IS fighters who already controlled most of the Anbar provincial capital used a wave of suicide car bombs Thursday and Friday to take over most of the city.
Army, police, counter-terrorism and local tribal forces had been confined to the operations command base on the northern bank of the Euphrates and the large judicial compound facing it.
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Police Colonel Jabbar al-Assafi said government forces had withdrawn from Ramadi city.
"The security forces -- army and police -- have pulled out of Ramadi. They headed to the main highway," west of the city, he told AFP by telephone.
An army lieutenant colonel who was among the troops that pulled out of the operations command centre confirmed that IS now controlled all main security bases in the city.
"Army and police and all forces that were stationed at the Anbar operations command have pulled out, as they have from the nearby police station and judicial compound," he said on condition of anonymity.