By Roland Buerk
BBC News, Colombo
People are leaving their village homes
Up to 75,000 people have fled their homes in northern Sri Lanka in the last two and a half months, the UN says.
This adds to huge numbers of people already displaced by years of war in the troubled island.
Government forces are advancing into territory controlled by the Tamil Tiger rebels, who want an independent state for the island's ethnic Tamil minority.
The UN is calling for continued access to those who have been displaced to avoid a humanitarian crisis.
Sri Lanka's government does not allow journalists to go to the areas where the conflict is taking place.
But what is clear is that the military is now moving forward at a rapid pace.
People recently displaced are living in the open air under trees
For months the fighting in the north was taking place on largely static front lines. Now village after village, and rebel base after rebel base, according to the ministry of defence, is falling into government hands.
On Wednesday soldiers took control of Mulankavil, which the ministry of defence described as a Tamil Tiger garrison town.
It added that the forces were now within 15km of Kilinochchi town, the rebels' administrative hub.
The UN says as many as a third of the entire population of the rebel areas have abandoned their homes since the start of June.
"I think a lot depends on what happens over the next few weeks," said Neil Buhne, the UN Resident and Humanitarian Coordinator in Sri Lanka.
"If we are able to have continued access in terms of humanitarian supplies to these people, if there is safe space for the civilians in that area, if both sides respect their responsibility to allow displaced people to move where they want to move they I think a crisis can be avoided."
Mr Buhne estimated between a quarter and half of the people recently displaced are living in the open air under trees.
Sri Lanka's government says it is on the brink of victory in a war that began a generation ago.
Last July it announced the Tigers had been driven from areas they controlled in the east of the island.
Provincial elections have since been held there.
Now the forces are advancing into the north.
It is impossible to get independent confirmation of the military's progress, and the high casualties they claim to be inflicting on the rebels.
But people living in Tiger-held areas say large numbers of funerals are taking place, coffins draped in rebel flags carried on the back of pick up trucks to graveyards.
And they speak of a palpable slump in morale, even though the Tigers have reversed military advances in earlier phases of the war.
On their website the Tigers wrote that indiscriminate bombing and shelling is forcing people to leave their settlements.
And they issued an urgent call to people in areas they control to dig bunkers in their homes and workplaces.
"The [ethnic majority] Sinhala nation is intent on occupying and enslaving the Tamil homeland," the rebels said in one statement.
"Our military is only involved in a war of self defence against this war of the Sinhala nation."
Many Tamil civilians are trapped between the warring sides
But the government insists Tamils being displaced now will be better off if the rebels are defeated.
"We are on a liberation activity," said Keheliya Rambukwella, the government's defence spokesman.
"We will liberate the north, sooner than people even think. When you need to look at a bigger target, a bigger goal, you need to make certain sacrifices down the line. And these are the sacrifices and for these sacrifices we are not going to let them down."
The government alleges the Tamil Tigers could be planning to use civilians in the north as human shields.
Very few have crossed into government-controlled areas, but the United Nations says there has no been no prevention of movement to keep them in Tiger-held territory.
The displaced people are converging on Kilinochchi likely to be a key battleground as Sri Lanka's government continues to try to win the war.