BP remains in the firing line in the US despite apparent progress in capping the Gulf of Mexico oil leak.

The oil giant faces anger over the refusal of outgoing chief executive Tony Hayward to attend a Senate committee and reports say that a criminal enquiry into the spill has been launched.

A US Senate hearing into the release of the Lockerbie bomber and BP's possible role in the affair has been cancelled after Hayward, Scottish Justice Secretary Kenny MacAskill and former UK Justice Secretary Jack Straw refused to attend.

Many in the US believe that the decision to release Abdelbaset Ali Mohmed Al Megrahi, who was jailed in Scotland for his part in a plane bombing that killed 270 people in 1988, was linked to BP's desire to bag lucrative oil contracts in Libya.

The refusal of the BP executives to attend the hearing provoked an angry reaction from Democratic senator Robert Menendez.

'We are at a place where no witnesses of consequence have the courage to step forward and clear the air,' he said.

'They would prefer to sweep this under the rug.'

The renewed anger against BP comes as a cap to prevent oil leaking from the damaged well appears to be holding.

But there is still uncertainty as to whether the latest measures will be successful and according to the Washington Post, a team of investigators from US environment, coast guard and other agencies are conducting a criminal enquiry into the spill.

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