US Under-Secretary of State Thomas Pickering talks to the BBC's Martin Turner in Abuja

In an interview with the BBC, Mr Pickering said it was important for Nigeria that the exact cause of Chief Abiola's death was established, although he said there was no evidence so far to suggest foul play.

Earlier he described the dramatic last few hours of Chief Moshood Abiola's life.

Mr Pickering was in Nigeria with a delegation of US diplomats to discuss the country's planned transition to democratic rule.

Mr Pickering said: "Part way through meeting, in the early stages, he asked to restore his thought and took some tea.

"He had trouble breathing, went into the toilet and came out obviously very distressed. He asked for both cough medicine and painkiller and then sat on a couch.

"He was clearly overheated and removed his shirt and asked that the room be ventilated and then generally declined.

"Susan Rice (one of the US delegation) immediately asked the security officer who was with us to send for a physician."

Race to hospital

Mr Pickering told the American CNN channel that the doctor arrived in about 10 minutes and diagnosed that Chief Abiola needed immediate hospital attention.

He said: "We all helped to put him in a car - there was no ambulance immediately available.

"We followed him to the clinic of the head of state of Nigeria, where doctors immediately began to work on him.

"In an attempt to save his life they worked fully for 90 minutes in all conceivable ways.