Relief workers are battling to overcome transport and communications problems to bring aid to about one million Sri Lankan homeless.
There is "frighteningly little aid" in some areas, experts say
A UN team has arrived to assess the magnitude of the disaster.
However, many areas are still receiving only a trickle of supplies. A total of 21,715 people are now confirmed dead.
Tamil Tiger rebels, who control areas in the north and east, have accused the government of failing to divert aid there, a charge the government denies.
A web site at www.contactsrilanka.org has been set up by the government to provide disaster information.
The BBC's Dumeetha Luthra in the southern town of Galle, says a slow trickle of aid is arriving there, with a few relief centres set up.
But the magnitude of the disaster has made it difficult for the UN team to provide a fast, coordinated response, our correspondent says.
The country has yet to see an organised international effort get off the ground, she says.
Chris Weeks, of the private Disaster Resource Network in Colombo, says there is still "frighteningly little aid" in the coastal towns worst hit.
"There seems to be a lot of people who have turned up but not much in the way of tents and blankets and medical
equipment," he said.
The nation's one million homeless are in urgent need of shelter, drinking water and basic rations.
There are reports of outbreaks of measles, diarrhoea and other diseases and fears that they could develop into epidemics.
Urgent efforts are being made to bury the thousands killed by Sunday's sea surges to prevent the spread of the diseases.
Bodies were being photographed, fingerprinted and buried quickly.
Government relief co-ordinator, Thilak Ranaviraj, said: "The most important thing is the quality of water."
President Chandrika Kumaratunga has called for national unity and Prime Minister Mahinda Rajapakse has promised aid will stretch to all areas of the country.
However, the Tamil Tiger rebels have complained that aid is not reaching areas under their control.
They made a separate appeal, calling for $2m of international assistance.
The Tamil Rehabilitation Organisation appealed for milk powder, clothes, drugs, bandages and water purification
There were unconfirmed reports from Sri Lankan officials in the east that four trucks of sugar bound for Tiger-held areas were diverted by mobs and officials to Sinhalese areas.
The government insists its relief packages are being distributed fairly.
However, despite the differences, Tigers leader Velupillai Prabhakaran said in a message: "My condolences also go to our Muslim and Sinhala brethren in the southern coastal areas, who have lost their kith and kin and are in deep sorrow."
Reports that landmines have been dislodged in the north and east and are hampering aid efforts have been denied by mine agency officials.
The train disaster at Telwatta could be the world's worst
Charles Frisby, from the anti-landmine charity, the Halo Trust, said there were no confirmed reports in Jaffna and Trincomalee of dislodged mines.
"Relief agencies must press on with their work," he told the BBC.
On the south and east coasts, damage to communications and transport are severely hampering relief efforts.
The World Bank says about 800km (500 miles) of railway line has been destroyed.
A packed train travelling between Galle and Colombo was caught in the sea wave on Sunday at Telwatta and swept off the tracks.
Reports say more than 1,000 people may have died, which would make it the world's worst rail disaster.
Jeff Dick, UN World Food Programme director in Sri Lanka, said: "Communication lines remain extremely problematic, and many key logistic routes needed to transport food have been blocked."
Meanwhile the Sri Lankan Tourist Board has set up a dedicated website to provide information for those concerned about relatives or friends, with hotline and emergency service numbers.
Efforts continue to repatriate foreign tourists.
Tourist board chairman, Udaya Nanayakkara, said most group-travel tourists had been evacuated and now independent travellers were being collected.