The United States is a hard country to defend, President George W Bush admitted on Wednesday.

"Our intelligence is good. It's just never perfect..." Bush told executives of over 1,500 Associated Press-member newspapers at the cooperative's annual meeting in Washington, DC.

"We are disrupting some cells here in America. We're chasing people down. But it is a big country."

Asked about a poll showing that 67 per cent of Americans thought it was likely that a terrorist attack would be carried out in the between now and the November 2 presidential polls, Bush referred to last month's train bombings in Madrid.

"I can understand why they think we're going to get hit again," Bush said. "They saw what happened in Madrid."

The Madrid train blasts, which killed 191 people and injured more than 2,000, took place three days before the general election in Spain, and were widely seen as contributing to the ruling party's unexpected defeat.