He said the cartels were also seeking tip-offs about police investigations.

Another official said 127 US agents had been arrested or tried since 2004.

Alan Bersin, the US customs and border protection commissioner, said Mexico's offensive against the cartels combined with the rise in the hiring of border agents in recent years has multiplied the risks of corruption.

Mr Edwards named the Zetas cartel as being involved increasingly in systematic corruption.

It came in the form of monetary bribes, sexual favours and other methods to encourage border agents to assist drug traffickers, those involved in smuggling undocumented immigrants, or to ignore their activity, he said.