By Laura MacInnis GENEVA, Feb 10 (Reuters) - More than half the 250,000 civilians now trapped in Sri Lanka's war zones may soon flee their homes and require international assistance, the U.N. refugee agency UNHCR said on Tuesday.

Thousands of people in the past week have fled intensifying fighting between Sri Lankan troops and the Liberation Tigers of Tamil Eelam (LTTE) in the past week, with the government seeking to end a civil war that has flared on and off since 1983.

The separatist rebels are now confined to less than 175 square km (67 sq miles) in the Indian Ocean island's northeast.

Ron Redmond, a spokesman for the U.N. High Commissioner for Refugees, said aid groups were mobilising assistance to help the people penned in by the violence who may seek to escape the area in large numbers.

"UNHCR and its partners are prepared to meet the immediate needs of up to 150,000 civilians fleeing from the conflict zones ... into government-controlled areas," he told a news briefing in Geneva, where the agency is based.

Redmond called on both Sri Lankan government and rebel forces to "halt indiscriminate fighting in close proximity to large concentrations of innocent people".

Sri Lanka's military said Tamil fighters gunned down 19 people seeking to flee the war zone on Tuesday, the day after a suicide blast at a refugee registration centre that it blamed on a female LTTE fighter dressed as a civilian.

The Tigers could not be reached for comment, because most communication to the war zone has been cut off.

But the pro-rebel site website www.TamilNet.com on Tuesday accused the military of shelling civilian areas, including a military-demarcated no-fire zone, killing at least 36 people. TamilNet also accused the military of shooting civilians after Monday's suicide blast. The military denies both claims.

Referring to Monday's attack, Redmond said innocent people in search of safety needed to be better protected.

"We also call on the LTTE to allow the remaining civilian population trapped inside the conflict zone to move to areas where they feel safe," he said.

To date, the UNHCR said more than 13,000 people have fled the conflict areas and sought international assistance in the Vavuniya, Jaffna, and Mannar regions. Another 5,000 are expected to reach Vavuniya in the next couple of days, Redmond said.

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