By C. Bryson Hull

COLOMBO, Jan 30 (Reuters) -- Pressure rose on Friday for Sri Lanka and the Tamil Tiger rebels to free thousands of people trapped in the war zone, after the president pledged safe passage and urged the rebels to let people move over the next two days.

Sri Lanka's army has surrounded the separatist group in a 300 sq km (115 sq mile) slice of jungle in the Indian Ocean island's northeast, gunning to end a war that started in 1983 and now is one of Asia's longest-running conflicts.

Aid agencies say 250,000 people are trapped inside the battle zone, and have raised grave concerns for their safety.

The International Committee of the Red Cross (ICRC) has said hundreds have been killed or wounded in the fighting since last week. The government says that the reported numbers are too high, but has not provided an exact figure.

Australia's Foreign Minister Stephen Smith on Friday joined a growing international chorus urging swift action.

"Australia holds grave concerns for the large number of civilians who remain in the conflict zone. There is an urgent need to ensure their protecion," Smith said in a statement, that also welcomed the government's promise of safe passage.

On Thursday, President Mahinda Rajapaksa in a statement promised safe passage to all civilians, and urged the Liberation Tigers of Tamil Eelam (LTTE) to let them go within 48 hours.

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 a United Nations-ICRC convoy on Thursday that carried out 226 seriously wounded people.

The United Nations said the LTTE had blocked the convoy from leaving two days before, which the Tigers denied.

"We call upon the LTTE to allow full freedom of movement to all civilians, and to allow safe passage to those wishing to leave the conflict area," a consortium of aid agencies including the United Nations said in a statement released late on Thursday.

The groups also asked the government to advise people on safe routes out with international assistance "that would provide safety measures for civilians coming out of the conflict zone seeking protection."

The government and human rights watchdogs say the LTTE has blocked civilians from leaving, instead forcing them to stay as human shields, fighters or labourers building defences.

The LTTE denies that, and says people are staying of their own free will because they fear abuse by the military. The military denies abusing anyone and says that more than 2,000 people have fled to safety behind army lines this month.

Both sides have traded blame for the casualties.

The military says the Tigers are firing artillery from populated areas inside an army-declared no-fire zone with the hope of creating a crisis to build pressure for a truce, as they have done in the past when losing on the battlefield.

The Tigers deny that and have continually accused the military of firing into the no-fire zone. The military denies that, and says it has a policy of zero civilian casualties.

It is nearly impossible to verify accounts from the war zone, since journalists are usually barred from it except for when they are allowed in on carefully guided tours. (Editing by David Fox)

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