The American Embassy in Baghdad plans to evacuate a substantial number of its personnel this week in the face of a militant advance that rapidly swept from the north toward the capital, the State Department announced on Sunday.

The embassy, a beige fortress on the banks of the Tigris River within the heavily-secured Green Zone, where Iraqi government buildings are also located, has the largest staff of any United States Embassy.

The exact number of people being evacuated was not clear Sunday. The embassy would remain open, a person familiar with the planning said, and much of its staff of about 5,500 would stay in Baghdad. The American government is expected to call the move a relocation, suggesting that it is a temporary precaution, the person said.

Many staff members who are leaving will be flown to Amman, Jordan, where they will continue their work at the embassy there. Others will be shifted from Baghdad to consulates here in Erbil, in the northern Kurdish region, and in Basra, in the south, which are not now under threat by the militants.