WASHINGTON, Sept 23: Homeland Security official Asa Hutchinson on Thursday said the United States was on heightened alert for terrorist threats, particularly involving aircraft, ahead of the Nov 2 presidential elections.

The remarks by Mr Hutchinson, the department's under secretary for border and transportation security, came just days after Yusuf Islam, the singer formerly known as Cat Stevens, slipped onto a US-bound flight unnoticed despite his being on a government watch list.

"We are very concerned about this election year and the threat that it poses and the aggressive attitude of the terrorists. "And aviation is something they have historically looked at, so we're very alert. We are taking the precautions that we should be, and we ask the American public to understand the reasons for these security measures," he said.

Mr Hutchinson said officials were "distressed" that the singer, who changed his name after converting to Islam in 1977, was on the Washington-bound United Airlines flight that was ultimately diverted to Bangor, Maine, once his presence on board was noted.

"The system did not work, because he should not have been allowed to get on the plane," Mr Hutchinson told Fox News. He noted that intelligence officials had determined Islam to be a threat but declined to be more specific about the type of threat he allegedly posed.

"We are not going to go into what he did or what the intelligence was, but the rules are that there has to be ... a connection to some type of terrorist activity. There has to be a risk to civil aviation," Mr Hutchinson said. Islam was denied entry to Israel in 2000 over suspicions that he had given money to the Hamas. -AFP