(Adds president's quotes)

By C. Bryson Hull

COLOMBO, Jan 29 (Reuters) - Sri Lanka's president on Thursday pledged safe passage for thousands of people trapped by fighting with Tamil Tiger rebels, hours after a convoy carrying more than 200 seriously wounded people left the war zone.

Amid rising international outcry over the fate of civilians, President Mahinda Rajapaksa challenged the Liberation Tigers of Tamil Eelam (LTTE) to let people leave the 300 square km (115 sq miles) of jungle where the separatist rebels are now cornered.

"I urge the LTTE within the next 48 hours to allow free movement of civilians to ensure their safety and security. For all those civilians, I assure a safe passage to a secure environment," Rajapaksa said in a statement.

The military said it would not cease combat operations or declare a ceasefire, but would stop shooting to allow civilians to get out as it did for the convoy on Thursday.

"Whenever civilians want to move into safer areas, we will adhere to that," spokesman Brigadier Udaya Nanayakkara said.

Sri Lankan troops are battling to finish one of Asia's longest wars, having won a series of major battles and swiftly encircled the LTTE in the northeast of the Indian Ocean island.

Aid agencies say about 250,000 civilians are caught between the foes and at serious risk.

Rajapaksa's announcement came after United Nations and International Committee of the Red Cross (ICRC) said a convoy took 226 people out of the war zone after days of negotiations, while another ferried in 170 tonnes of food.

The ICRC had said hundreds have been killed or wounded in the last week. The government says those numbers are too high and may include wounded rebels. It says it has no exact figure.

NO EXODUS

Many people were stranded without adequate treatment in poorly equipped hospitals until the U.N.-ICRC convoy carried them to a government hospital in Vavuniya, outside the war zone, the ICRC said.

"The wounded and sick had to wait for days before being transferred safely. It had become critical that they receive medical treatment immediately," Paul Castella, head of the ICRC delegation in Sri Lanka, said in a statement.

The United Nations had said it tried to get the people out on Tuesday, but the LTTE stopped them from going.

On Thursday, LTTE political head B. Nadesan denied the rebels blocked the convoy, pro-rebel web site www.TamilNet.com said.

"Any individual who wishes to leave the combat zone has an individual right to move anywhere," TamilNet quoted him as saying. "But we will not be organising any 'organised exodus' against the collective will of the people."

Human rights watchdogs and the government accuse the LTTE -- designated a terrorist group by India, the United States and the European Union -- of keeping civilians in its territory to use them as fighters, battlefield labourers or human shields.

Both sides have traded blame for the casualties, with the Tigers saying the army is shelling a no-fire zone it set up last week for civilians.

The military denies that and says the Tigers moved artillery to keep people from going into that area. It also accused the LTTE of creating a civilian crisis to build pressure for a truce, as it has done in the past when losing in battle.

It is nearly impossible to verify accounts from the war zone, since journalists are rarely allowed in. (Additional reporting by Stephanie Nebehay in Geneva) (Editing by Richard Meares)

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