DHAKA, April 29 (Reuters) - Bangladesh on Tuesday asked farmers to speed up their rice harvest as the country braces for a powerful cyclone likely to hit in two or three days.

Cyclone Nargis is about 1,200 km (750 miles) southwest of Bangladesh's Chittagong coast and likely to gain strength in the Bay of Bengal as it nears the coast.

British-based storm tracking service Tropical Storm Risk (http://tsr.mssl.ucl.ac.uk/) rated Nargis as a category 2 storm on Tuesday and it was expected to reach category 4 strength in 36 hours. Category 5 is the maximum category for tropical cyclones.

The Bangladesh government issued a local number 2 storm warning at Chittagong and other ports and urged residents along the coast to be ready for quick evacuation.

Bangladesh has been facing serious food shortages after last year's floods and a cyclone, which together damaged nearly 3 million tonnes of rice, the country's main staple.

Food stocks has been further affected by the army-backed interim government failing to import enough rice to fill the gap because of a global spike in food prices as well as limited supplies from exporters.

"We have advised farmers all over the country to harvest the boro (rice) crop as quickly as possible, if the crop is ready," said Habibur Rahman, a senior official at the Department of Agriculture.

"They need not wait unnecessarily ... as strong winds would likely damage the rice in the fields," he told Reuters.

Cyclone Sidr killed nearly 3,500 people last November. It destroyed 1.8 million tonnes of rice in the fields and left around two million people homeless.

The storm was the most powerful to hit Bangladesh since a cyclone in April 1991 killed 143,000 Bangladeshis.

Boro is a major rice variety in Bangladesh, which officials expect to yield around 17.5 million tonnes this year, two million tonnes more than last year.

If the harvest was completed without any losses, officials and economists hope food prices in markets, which nearly doubled over the past year, would start coming down.

But the Nargis was posing a new threat, Habibur said.

Bangladesh began harvesting boro about a week ago but the officials said it would take another fortnight to complete.

Most of the boro fields are far from the coast and officials said they hoped most of the rice fields would be largely spared by the storm.

"But we cannot take for granted as we saw the Sidr devastated many areas deep inside the country," one official said. "We have no option but to stay alert." (Reporting by Azad Majumder; Writing by Anis Ahmed; Editing by David Fogarty)

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