Holmes, the under-secretary general for humanitarian affairs, told government leaders that months of fierce fighting had taken "a terrible toll" on civilians, as international pressure grew for an immediate ceasefire.
The UN believes up to 50,000 civilians are trapped in a strip of jungle where Sri Lankan soldiers have surrounded the remnants of the once powerful Liberation Tigers of Tamil Eelam (LTTE).
"We need a new humanitarian pause to get aid and aid workers into the combat zone," Holmes said in a statement, adding that the Tiger guerrillas should "let out the remaining civilian population and lay down their arms."
Holmes, on a three-day visit to Sri Lanka, met Sunday with Human Rights and Disaster Management Minister Mahinda Samarasinghe and was due to see other leaders before leaving the island.
He has said he also intends to inspect the camps where 100,000 Tamil civilians have been detained since escaping the violence.
The government say the camps are necessary as