An aid ship which arrived off the northern port of Jaffna on September 9 has only been able to unload its cargo slowly using smaller shuttle craft, leaving it vulnerable to sea-borne attack, Berthiaume said.
The rebels who control the Jaffna peninsula and the city had refused to guarantee the vessel's security, she said.
The Tamil Tigers instead say that supplies should be brought in by road, something which Sri Lankan officials have opposed.
"On one side you've got the Tamils who are stopping the boat and on the other the Sri Lankan defence ministry which is preventing us from reaching Tamil-controlled areas," said Berthiaume.
"We are in an impasse over Jaffna," she added.
Earlier Tuesday, the Sri Lankan government had accused international aid agencies of chickening out when civilians needed help the most amid fierce fighting between troops and rebels.
The main land access to the region has been cut off by fighting which erupted a month ago and supplies are running dangerously low in Jaffna for the 350,000 civilians there, according to aid workers.
Two weeks ago, relations between the government and aid agencies hit a new low with truce monitors accusing security forces of killing 17 local employees of a French charity in the island's northeast.
The government has denied killing the aid workers.Relief workers say they cannot operate without both sides agreeing to guarantee them safe passage.