(Adds comment from military official, detail)
By Jared Ferrie
SITTWE, Myanmar, May 14 (Reuters) - A boat carrying about 100 Rohingya Muslims capsized off western Myanmar with many feared drowned at the start of a mass evacuation from low-lying regions ahead of an approaching storm, a U.N. official said on Tuesday.
The boat struck rocks off Pauktaw township in Rakhine State and sank late on Monday, Barbara Manzi, head of the Myanmar office of the U.N. Office for the Coordination of Humanitarian Affairs (OCHA), told Reuters.
She said an unknown number of people were missing.
A military intelligence officer told Reuters at least 50 people drowned when the boat went down at around midnight. It was one of six leaving Pauktaw, said the official, who declined to be identified because he was not authorised to speak to media.
Kirsten Mildren, a spokeswoman for OCHA in Bangkok, said she understood there was one big boat with an engine towing two smaller, wooden boats without engines, and that between 100 and 150 people were on the three vessels.
"We understand that yesterday evening they went out with the approval of government officials. This was part of an official government evacuation plan although the boats were not government boats. They were moving from a low-lying area to a safer area," she said.
The approaching storm is a tropical depression named Mahasen which is expected to strengthen into a cyclone.
Forecasts by the U.S. Navy's Joint Warning Center show the storm making its way north over the Bay of Bengal. It is expected to make landfall on Thursday near Chittagong in Bangladesh before moving into neighbouring Myanmar.
It threatens a region of Myanmar where about 140,000 victims of ethnic and religious unrest are living in camps. The United Nations warned last week that Myanmar could face a "humanitarian catastrophe" if people were not evacuated.
The United Nations said about 69,000 people, most of them Rohingya Muslims, were living in "precarious" conditions at risk of flooding and other damage during the rainy season, which begins this month and continues until around September. Mahasen could bring "life-threatening conditions", it said.
The evacuations, a combined effort between the government and aid agencies, are seen as a test of Myanmar's willingness to assist stateless Rohingya Muslims, an impoverished and long-persecuted people who bore the brunt of sectarian violence in Rakhine State last year.
Myanmar is a predominantly Buddhist country but about 5 percent of its 60 million people are Muslims. They face a growing anti-Muslim campaign led by radical Buddhist monks.
Evacuations began on Monday in some areas such as Pauktaw, a port town about 27 km (17 miles) by sea from the state capital Sittwe. Nearly 20,000 Rohingya were living in camps in Pauktaw after last year's violence, including about 12,000 in flood-prone areas, according to U.N. data.
At least 192 people were killed in June and October last year in unrest between ethnic Rakhine Buddhists and Rohingya Muslims who are denied citizenship by the government in Myanmar and considered by many there to be immigrants from Bangladesh.
Hundreds of Rohingya homes were destroyed in Pauktaw in October.
In Sittwe, up to 45,000 displaced people living in low-lying areas have been identified as the most vulnerable to Mahasen. Some have begun to be moved to higher ground.
In Bangladesh, authorities told residents of outlying islands to start evacuating. Large vessels were told to remain at anchor in outer areas, while fishing boats and other smaller boats were warned to remain near the coast and not venture out to sea.
Officials in the Bangladeshi town of Cox's Bazar near the border with Myanmar said medical teams with as many as 30,000 Red Crescent volunteers were being formed.
In eastern India, authorities put 10 coastal districts on alert.
Images taken by NASA's Aqua satellite on May 13 showed the storm's centre northeast of Sri Lanka with it packing winds of up to 50 knots (92 km per hour/57 miles per hour). Those winds are expected to increase to 130 km per hour (80 mph) as the storm moves north.
The space agency said it "sees a strengthening" of the storm and forecasts an upgrade to a Cyclone 1 level by Wednesday.
In 2008 a cyclone swept across Myanmar's Irrawaddy Delta, south of the old capital, Yangon, killing up to 140,000 people. (Writing by Jason Szep; Additional reporting by Soe Zeya Tun in SITTWE, Amy Sawitta Lefevre in BANGKOK, Serajul Quadir in DHAKA and Annie Banerji in NEW DELHI; Editing by Alan Raybould and Robert Birsel)