"We believe that there are critical levels of hunger, and large numbers of people needing medical treatment."
The UN estimates that up to 50,000 non-combatants are still in the conflict zone, although the government maintains that the number is less than 20,000.
Holmes issued the statement after a two-day visit to Sri Lanka, where he met Mahinda Rajapkase, the Sri Lankan president, in an effort to bring a halt to the fighting.
But David Chater, reporting for Al Jazeera from Colombo, the Sri Lankan capital, said that the UN official had not managed to secure access to the combat zone for a small team from the world body.
Holmes' attempt to get access to the conflict zone has been rebuffed by Colombo [AFP]
"Absolutely nothing has changed as a result of John Holmes' visit, apart from another ten million dollars in humanitarian aid being pledged," he said.
"[That money could provide] at least a bit of relief for those who got out of the combat zone, but no relief for those still inside."
The US also voiced concern on Tuesday about the condition of civilians believed trapped in the conflict zone.
"We're still very concerned about ... unconfirmed reports that heavy shelling has continued in the conflict zone," Robert Wood, a state department spokesman, said.
"We want to make sure that the government of Sri Lanka and the LTTE do their utmost to protect civilians in the conflict area. They need to live up to their word."
With the Sri Lankan government tightening its grip on the country's north and east, Vinayagamoorthy Muralitharan, a former LTTE senior military commander, has told Al Jazeera that the Tamil Tigers will fight to the end.
"I think this is LTTE's last moment, because they will never build up again," he told Chater in an exclusive interview.
But Muralitharan, who was once known as "Colonel Karuna", said Velupillai Prabhakaran, the LTTE chief, would not surrender.
"He has a totalitarian policy. He never changes from that policy, he totally believes that policy.
"But according to my knowledge, he will never surrender. He will never surrender to the army because he knows his crimes."
With the LTTE confined to a tiny strip of territory in the island's northeast, Prabhakaran's fate is shrouded in mystery.
The Sri Lankan military has ordered its troops to end the use of heavy weaponry and aerial bombardment, in what it called an effort to ease the suffering of civilians.
But a pro-Tamil Tiger website on Tuesday accused the military of continuing to shell areas of the conflict zone populated by civilians.
Speaking to Al Jazeera, Thileevan, an LTTE spokesman inside the conflict zone, said the area had been shelled heavily.
"We don't know how many people were killed because we could not get out of this area. But when I went to the hospital this morning I saw hundreds of severely wounded people," he said.
"We ask the international community to intervene in this problem and save our people ... We [the LTTE] carry weapons to save our people and protect their rights."
The military has denied the accusations.
In the latest setback to international efforts to end the conflict, Sweden's foreign minister said on Tuesday that he had been refused entry to Sri Lanka on a European mission aimed at bringing about an immediate ceasefire.
Carl Bildt was due to visit the country on Wednesday with his British and French counterparts, but he told the Associated Press news agency that Sri Lankan authorities did not give him permission to enter the country.
David Miliband, the British foreign minister, and Bernard Kouchner, the French foreign minister, will be allowed into the country, Bildt said.
Colombo says it is on the verge of defeating the LTTE after 37 years of conflict, and has consistently brushed off international calls for a truce.
It rejected on Sunday an LTTE call for a unilateral ceasefire.