The United States Navy has begun what is expected to be the last round of military exercises on the Puerto Rican island of Vieques.

Navy spokeswoman Lieutenant Commander Kim Dixon said the first day of the exercise, which began at 1030 am (1530 GMT), involved aerial bombing from fighter jets using inert explosives.

The Vieques issue has strained US-Puerto Rico relations

When the training ends in about a month's time, the protesters who have long campaigned for a end to the US military presence will finally get their way.

But the protests continued on Monday, with Navy guards detaining five demonstrators who had broken through a security fence - one of whom shouted "Peace for Vieques!" as guards led him away.

The detainees were identified as members of the Puerto Rican Independence Party.

"If the Navy is leaving, it is precisely because of people going in to disrupt manoeuvres," said Independence Party Senator Fernando Martin. "It is the essence of sound judgment to continue to apply pressure."

Linked to cancer

The military held its first training on Vieques in 1947, and has used its range on the island's eastern tip ever since, to train sailors for conflicts ranging from Korea to Afghanistan.

About 8,000 sailors are participating in the latest exercise, many of whom are preparing for deployment later this year to the Mediterranean.

Vieques' 8,000 residents have long objected to the use of their island as a bombing range, especially as depleted uranium (DU) shells have been linked to soaring cancer rates on the island.

The protests intensified after a Puerto Rican security guard was hit by a stray bomb in 1999.

In 2001 the Bush administration finally pledged to end military activity on the island, and the Navy says training will cease from May.

It says bombing ranges in Florida as well as a computer simulation training system known as "Virtual Sea" will be used instead.

The US may also close Roosevelt Roads naval station in east Puerto Rico, which is the largest employer on the island.

"Without Vieques there is no way I need the facilities at Roosevelt Roads - none. It's a drain on defence department and taxpayer dollars," Admiral Robert Natter, commander of the Atlantic Fleet, told the Associated Press.

But pro-Navy activist Luis Sanchez, who set up a small encampment just outside the Navy's fences, said: "If they abandon Vieques and Roosevelt Roads, it will destroy the economy."