Myanmar on Tuesday sentenced seven Muslims to prison terms ranging from two to 28 years in connection with religious violence in March that left dozens of people dead, a justice official said.

The defendants, who were spared the death penalty, were accused of the murder of a Buddhist monk in the central town of Meiktila that sparked unrest across the region, mostly targeting Muslims.

U.S. President Barack Obama on Monday threw his support behind Myanmar President Thein Sein in his drive to reform a former pariah state but warned that a wave of violence against Muslims must stop.

As his guest became the first leader of his country in almost 50 years to visit the White House, Obama praised Myanmar's journey away from brutal junta rule and promised Washington would offer more political and economic support.

Myanmar President Thein Sein began Saturday the first visit to Washington by a leader of his country in nearly 50 years as the United States throws its support behind his reforms.

The former general, who initiated a wave of reforms after taking office in 2011, flew into Washington and was holding a weekend of private meetings before talks at the White House on Monday, people involved in the trip said.

Myanmar released about 20 political prisoners on Friday, a top official said, hours before its reform-minded leader was due to leave on a landmark visit to the United States to meet President Barack Obama.

President Thein Sein, a former general, has freed hundreds of political detainees since coming to power in early 2011 as part of sweeping changes that have led to the end of most Western sanctions.

President Thein Sein's historic invitation to the White House is an endorsement of "Myanmar's Spring" and a further sign that the former pariah's reforms are irreversible, a senior Myanmar official said.

Washington will welcome the former general on Monday in a hugely symbolic reward for sweeping changes since he took power two years ago. He will be the first leader of the former military-ruled nation to visit since 1966.

The White House said it will welcome Myanmar's leader on a landmark visit on Monday in a symbolic reward by President Barack Obama to encourage reforms in the longtime pariah state.

President Thein Sein, a former general who surprised even many critics by ushering in democratic changes, will be the first leader from the country formerly known as Burma to visit Washington since 1966.

Five foreigners who were kidnapped last month when heavily armed pirates stormed their cargo ship off Nigeria's oil-producing southern coast have been released, the vessel's German operator said Tuesday.

The Gulf of Guinea, which includes the waters of Benin, Nigeria and Togo is an emerging piracy hub, with gunmen frequently targeting oil ships both to steal crude and seize foreign hostages in order to get ransom payments.

A boat carrying up to 150 Rohingya Muslims fleeing a cyclone has capsized off Myanmar's coast, the U.N. said Tuesday, heightening fears over the storm which threatens camps for tens of thousands of displaced people.

The boat hit trouble on Monday night after it left Pauktaw township in Rakhine state, said a spokeswoman for the U.N. Office for the Coordination of Humanitarian Affairs, adding there were an unknown number of missing as "they were traveling to another camp ahead of the cyclone".

Myanmar's president will soon make the first state visit to the United States by a leader of the former pariah nation in almost half a century, state television announced Monday.

It did not say exactly when former general Thein Sein -- whose quasi-civilian government has won international plaudits for its political reforms since taking power two years ago -- would travel to Washington.

Myanmar's president on Monday pledged to uphold the "fundamental rights" of Muslims in strife-torn Rakhine state, in the wake of deadly religious unrest that has spread across the country.

In a speech to the nation following the release last week of an official report into last year's violence in western Rakhine that killed around 200 people, Thein Sein said the country should aim for "peaceful coexistence".