Image caption US Navy Seals are an elite special operations force

The US is to tighten security around the elite military unit that killed Osama Bin Laden, US Defence Secretary Robert Gates has said.

Mr Gates revealed that the US Navy Seal team had expressed concerns over their safety and that of their families.

Al-Qaeda has vowed to avenge the death of its leader in the helicopter-borne raid on his compound in Abbottabad, northern Pakistan.

Mr Gates said threats against US forces had increased since Bin Laden's death.

The defence secretary, addressing US marines at Camp Lejeune in North Carolina, said he had met the Seal team last week and they raised the issue of their security "and particularly with respect to their families".

Obama grandmother fears

"We are looking at what measures can be taken to pump up the security," he said.

"As a result, there has been a consistent and effective effort to protect the identities of those who participated in the raid. I think that has to continue," he added.

The team that attacked the Abbottabad compound is widely reported to have been the US Navy's Seal Team Six (ST6) - a secretive hand-picked unit. Seal is an acronym for sea, air and land.

Journalists have descended on Virginia where the Seals are based to try to glean more details of the unit and retired Seals members are in high demand as guests on news broadcasts.

Mr Gates said details of the raid had leaked despite the intention by officials to keep them secret.

"Frankly a week ago Sunday, in the [White House] situation room, we all agreed that we would not release any operational details from the effort to take out Bin Laden," he said.

"That all fell apart on Monday, the next day."

Meanwhile, Kenya says it is stepping up security around the home of President Barack Obama's step-grandmother, Sarah Obama, in the wake of Bin Laden's death.

"All visitors going to the home will have to be thoroughly vetted," said regional administrator Francis Mutie.

"They will undergo security screening before they are cleared to visit the home."

Sarah Obama, who lives in western Kenya, was the third wife of President Obama's paternal grandfather, Hussein Onyango Obama.