A spokesman for the International Committee of the Red Cross (ICRC) said the ship had anchored 24km off the coast of the Jaffna peninsula on Friday while another vessel was preparing to take foreign nationals from the area to safety.
More than three weeks of fighting has created a humanitarian crisis in north and east Sri Lanka.
Around 180,000 people have fled their homes and road, air and sea links to Jaffna have been cut.
Davide Vignati of ICRC said the ship did not want to enter the harbour at night.
"This morning the plan is to come in and unload," he said.
Most shops on the Jaffna peninsula have run out of food and some of its half a million people are down to one meal a day.
The ship is carrying over 1,000 tonnes of mostly dried food donated by the government and the World Food Programme.
Another small ferry flying the Red Cross flag would sail from the northeastern port of Trincomalee on Friday, officials said, taking to safety the first 150 of an estimated 500 expatriates, mainly Jaffna Tamils with British or Canadian passports.
Sporadic artillery fire continued from Trincomalee into separatist areas south of the port overnight, while in the eastern Batticaloa district the military said the Tamil Tigers clashed with elite police Special Task Force (STF) commandos.
The STF said they killed one separatist and one of their men died of his wounds.
Aid workers hoped fighting would subside on Friday to allow aid in.
The ICRC said that with security guarantees from both sides, they were confident their neutrality would be respected.