The ICRC told Al Jazeera on Wednesday, a day after the government declared victory over the LTTE, that "additional restrictions" meant it had no choice but to halt its activities.
Paul Castella, the head of the group's Sri Lanka operations, said: "Since last weekend, there have been additional restrictions imposed on aid organisations, including the ICRC.
"The authorities have said that because of security they had to restrict access to certain areas.
"What is the take of these civilians and what the conditions are we don't know because we are not granted access to the area."
According to Monica Zanarelli, the deputy head of operations for South Asia for the ICRC, aid trucks have been restricted from the largest camp since last weekend, bringing the distribution of supplies there to "a temporary standstill".
Until then, the Red Cross had delivered water, food, personal hygiene kits, baby-care parcels, emergency household items and kitchen utensils to the camp, known as Menik Farm, in the country's north, which housed more than 130,000 refugees, Zanarelli said on the Red Cross website.
Rishard Badurdeen, the Sri Lankan minister for resettlement, said access to the camps was restricted because LTTE fighters were hiding among the refugees.
"There are some 3,000 LTTE cadres in the camp and we haven't finished screening,'' Badurdeen told The Associated Press.
Zanarelli also said Red Cross officials had visited and registered more than 1,800 fighters who had surrendered.
The United Nations High Commissioner for Refugees (UNHCR) also reiterated calls for more assistance, citing the lack of services available for aid workers assisting the refugees who have left the former conflict zones.
"There are several issues that need urgent attention, including overcrowding and the limited services available at the camps,'' said Ron Redmond, the UNHCR spokesman.
"Civilians coming out of the conflict zone are sick, hungry and suffering from acute malnourishment and dehydration,'' he said in Geneva.
The Medecins Sans Frontieres (MSF) aid agency said thousands of refugees who had arrived in Vavuniya district are in "desperate need of medical care".
"A staggering 50,000 people have arrived in Vavuniya district since last Friday 16th May. Many thousands more are still expected in the coming days," the aid group said in a statement.
"Despite increasing the number of staff, MSF teams are overwhelmed by the huge and sudden influx of people."
Separately, rights groups have called on Sri Lanka to release three Tamil doctors detained after government forces overran the LTTE's last outpost in the country's northeast.
During the final stages of the conflict, journalists were prevented from entering the war zone and the international media came to rely on the reports of Thurairaja Varatharajah, Thangamuttu Sathyamurthi and V Shanmugarajah, three Tamil doctors working in the war zone.
The men were arrested on accusations that they gave false information about the number of casualties to the media. Police said that they can be held for up to 90 days.
Yolanda Foster, a Sri Lanka expert at the human rights organisation Amnesty International, told Al Jazeera: "Two of the doctors are now being held in the terrorist investigation division in Colombo, but no formal detention order has yet been issued.
"That's a key concern because without a formal order, the relatives of the doctors do not know where they are.
"If they are not to be charged formally then they should be released."
The third doctor is believed to be detained in hospital after being injured while escaping fighting on Friday. However, this has not yet been confirmed.
|Warnings continue to be issued of the dire humanitarian situation in refugee camps [AFP]
Mahinda Samarasinghe, Sri Lanka's minister for disaster management and human rights, confirmed that the doctors had been "handed over" to the police.
"The Criminal Investigation Department has issued detention orders for the three doctors and investigations have commenced about possible collaboration with the LTTE," he told Al Jazeera.
Mads Gilbert, a doctor at the Norwegian Aid Committee, told Al Jazeera that the doctors had fulfilled their humanitarian obligations and that it was "outrageous" that they had not yet been given full protection they are entitled to under international law.
"They should immediately be handed over to the UN or ICRC and be protected from any interrogation or punishment from the Sri Lankan government."
He said that the doctors should be afforded witness protection since they are some of the few who are able to talk of alleged war crimes by the government.
"They are instrumental to know what is going on... The most simple thing is for the government to execute them ... then they have silenced them completely," Gilbert said.
"These doctors are a threat to the government since they can tell the truth to the world."
Meanwhile, concern has grown over the high numbers of civilians killed and how the government will reach out to the country's Tamil minority.
The European Union has called for an independent inquiry into alleged human-rights violations carried out during the war.
The UN has said that between 80,000 to 100,000 people were killed in Sri Lanka's decades-long civil war.