The mass pull-out, demanded by the island's authorities for security reasons, has prompted fears for the fate of tens of thousands of Tamil civilians displaced by the military onslaught.
The Sri Lankan government said its soldiers were just over five kilometres (three miles) from Kilinochchi, the political capital of the Liberation Tigers of Tamil Eelam (LTTE).
"The fighting just kept getting worse... we pulled out all of our 35 staff and most of our equipment that we could take with us to government areas," United Nations spokesman Gordon Weiss told AFP.
Other international aid agencies like CARE and Germany's ZOA were also pulling out their staff, officials said.
Authorities have allowed only the International Committee of the Red Cross (ICRC) -- which supervises a civilian crossing point and carries out body exchanges for the military and the rebels --