Mr Adams said that he had been invited to the event and was "pleased to accept".

"When I arrived the staff at the White House informed me there was an issue of 'security'," he said.

"After two decades of travelling back and forth to the USA and countless meetings in the White House with successive US Presidents, this is an unacceptable development.

"It is obvious that there remain some within the US administration who seek to treat Sinn Féin differently."

Mr Adams added that Sinn Féin representatives had been denied entry or had to go through extra searches when travelling to the USA, while the State Department had also initially refused to meet him last year until "protest from US political leaders".

'Resolution'

"Sinn Féin will not sit at the back of the bus for anyone," he said.

"I am hopeful that the controversy around my White House invitation will help lead to a resolution to all these matters."

Mr Adams told the BBC that Sinn Féin representatives often face enhanced security checks but they normally do not "broadcast" it.

He said what had happened was "bad manners and not a good way to treat guests".

He said he had not been embarrassed by the incident, adding: "I just deal with this in a non-personal way."

Sinn Féin deputy leader Mary Lou McDonald said it was "ludicrous" for Mr Adams to be stopped at the White House for unspecified security reasons. BBC