Hanoi - Vietnamese workers found an unexploded bomb believed to have been dropped by the US Air Force during the Vietnam War, a military official said Monday.

The bomb, estimated to weigh 250 kilograms, was 1.2 metres long and has a diameter of 30 centimeters. It was found Friday in front of a house on central Hai Ba Trung street.

Workers were digging up the street to lay fibre optic cables when they discovered the bomb.

"We received orders from our commanders not to publicize the case," said Pham Van Khanh, commanding officer of the Hoan Kiem army steering committee. "Our commanders don't want people to panic that bombs still exist in the Hanoi city centre," Khanh added.

The bomb was transported to Hoa Thach Rifle Range on Saturday. An explosion was activated and it was destroyed safely.

Local media quoted Colonel Pham Duy Thinh, head of the Hanoi Military High Command of Engineers who said the bomb may have been dropped by the US between the years 1968-72, when the US escalated the war.

The bomb might have targeted Hanoi railway station, as infrastructure was targeted during the bombing campaign, leading to civilian losses.

US Senator John McCain, who referred to his experience as a prisoner of war during the presidential campaign, was shot down in 1968 while targeting a local power station. He was kept as a prisoner at the so-called "Hanoi Hilton," which is some 60 metres from where the bomb was found.

According to the United Nations, 104,000 Vietnamese have been killed by bombs, land mines and artillery shells since the end of the war in 1975.

In November 2008, the head of Vietnam's bomb disposal centre told a conference in Ho Chi Minh City that his centre estimated 800,000 tons of bombs, landmines and artillery are still buried across the country, in an area of 6.6 million hectares.

Between 15,000 and 20,000 hectares are cleared of unexploded ordnance each year. At the current rate it will take Vietnam another 440 years to clear all areas. (dpa)