Annan on Wednesday said UN workers had to rely on the occupation forces because the Muslim protection force proposed by Saudi Arabia for Iraq has not yet taken off.
The secretary general also pointed out that the proposal for a dedicated unit within the occupation forces to exclusively guard UN staff has not materialised with almost all countries reluctant to contribute troops.
"For the time being, for practical measures, we have no other choice but to rely on the multinational force and this is the way we are going," Annan said.
"We haven't had much success attracting governments to sign up for the dedicated force," he said.
A resolution adopted by the 15-nation Security Council in June called on the UN to play a vital role in helping Iraq down the path to its first democratically elected government and said a special unit of the occupation force should be set up to guard UN officials and facilities.
About 600 international UN staff were working in Iraq at one point after the US-led invasion.
But all were pulled out last year after a bomb attack on a UN office in Baghdad killed 22 people including the UN special representative for Iraq, Sergio Vieira de Mello.
Since then, only small teams have been sent in to Iraq to carry out special tasks while other senior UN officials worked on Iraqi issues from neighbouring Jordan.
The UN's new special representative for Iraq, veteran Pakistani diplomat Ashraf Jehangir Qazi is expected to return to Iraq with a small UN team in time for the 15 August national political conference seen as a crucial step toward January elections.