"And there is strong evidence that the LTTE (Liberation Tigers of Tamil Elam) are preventing them from leaving," said Holmes.
"I urge again all those with any influence on the positions of the LTTE to use that influence now to persuade them to let the civilian population go. There is no time to lose."
Holmes also said that the Sri Lankan government should "hold back from any final military battle in order to allow time for the civilian population to get out safely."
Sri Lanka's government has so far rejected calls for a ceasefire, saying they are on the verge of destroying the LTTE rebels and ending the 25-year-old civil war.
In the last weeks, troops have been driving the Tigers into a shrinking enclave in the country's northeast.
Tens of thousands of people have died in the civil war in which the LTTE are fighting for an independent homeland for ethnic Tamils.