* U.N. wants access to sick and wounded in former war zone

* Authorities blocking access to camps for displaced

By Jonathan Lynn

GENEVA, May 20 (Reuters) - The United Nations on Wednesday accused the Sri Lankan authorities of blocking access to civilians trapped in the former war zone or who have fled to camps for displaced people.

In the latest sign of concern at the government's treatment of ethnic Tamils and other civilians, the U.N. said it had no information on the numbers of sick or wounded still in the conflict zone.

"We need to have access, I repeat, total access, without the least let or hindrance, for the UN, for NGOs and for the Red Cross," Elisabeth Byrs, spokeswoman for the U.N. Office for the Co-ordination of Humanitarian Affairs, told a news briefing.

Byrs said NGOs (non-governmental organisations) were encountering difficulties getting into camps for displaced people, even though the military authorities in the Jaffna region had promised them total access.

An estimated 80,000-100,000 people were killed in 27 years of conflict between Tamil Tiger separatists and the government, she said.

U.N. Secretary-General Ban Ki-moon said on Tuesday he would visit Sri Lanka on May 22-23 for a first-hand look at displacement camps and the former war zone.

Earlier this week the government took control of all the island's territory for the first time since 1983, declaring victory in the conflict.

HUMANITARIAN NEED

The U.N. refugee agency UNHCR said it estimated that up to 80,000 people had left the former war zone in the last three days, bringing the total to have fled the fighting in recent months to 280,000.

UNHCR spokesman Ron Redmond said he did not know why the authorities were blocking access to the camps.

"It's urgent that assistance gets into those camps and that we are able to deliver. We've got lots of humanitarian supplies that need to be delivered," he told the briefing.

He said the government must improve conditions at existing camps, allocate public buildings to house new arrivals while UNHCR prepared land for new camps, deal with overcrowding at existing sites, uphold law and order in the camps, clarify policies for screening former combatants from civilians and give non-combatants freedom of movement.

Redmond said the government had the right to screen former combatants from fleeing civilians. Although UNHCR was monitoring the process it was not taking part close up.

"We want to see human rights upheld," he said.

Last month the International Committee of the Red Cross said the LTTE Tamil Tiger rebels were stopping civilians from leaving the war zone, while the government had prevented vital relief supplies from reaching the victims.

The U.N. Human Rights Council said on Tuesday it expects to hold a special session on Sri Lanka on May 25, but it is not certain that will go ahead.

Last week U.N. human rights chief Navi Pillay backed calls for an inquiry into possible war crimes in the fighting.

Ban met Sri Lankan Health Minister Nimal Siripala de Silva in Geneva on Tuesday, issuing a statement afterwards calling for access to all civilians. (Editing by Janet Lawrence)

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