Tamils have been fleeing the fighting in the north and east of the island
Sri Lanka's government has announced a ban on foreign aid workers and many of their local colleagues from working in Tamil-rebel held areas in the north.
Defence Secretary Gotabhaya Rajapaksa said Colombo could no longer guarantee the safety of aid workers in the area.
Colombo has intensified its war against the Tamil Tigers in a drive to crush their decades-old separatist struggle.
Aid agencies have been helping some of an estimated 160,000 people displaced by the fighting in the north.
They have in the past voiced concern for tens of thousands of people who have fled the frontline and sought refuge from the violence deep inside rebel-held territory.
The agencies have yet to respond to the government's announcement.
An estimated 85,000 people have fled their homes in the area since June, according to the United Nations.
Meanwhile, medical workers say 10 people have been injured in a blast in Colombo. A military spokesman told the BBC that the minor explosion appeared to have been caused by a grenade or homemade device.
One report suggested that gang rivalry was behind the attack, and that it was not linked to the island's civil war.
The government said the ban would apply to all foreign aid workers in rebel-held territory and to their local colleagues who were not permanently resident in the area.
"We can't assure the security of these people," Defence Secretary Rajapaksa told The Associated Press news agency. "We are taking precautions."
Mr Rajapaksa said any people affected by the ban who were currently in the area should leave immediately.
He said his government wished to avoid a repeat of the murder in 2006 of 17 local employees of French aid agency, Action Against Hunger.
Sri Lanka's government said Tamil rebels carried out the attack but international truce monitors said the killings were the work of the military.
A United Nations official last year described Sri Lanka as one of the world's most dangerous environments for humanitarian workers, prompting an angry rebuttal from the government.
The Tamil Tigers have been fighting for a separate homeland in the north and east for 25 years.
More than 70,000 people have died in the conflict.