Joe Biden, the Democratic vice-presidential nominee, has told Americans to expect a “major international crisis” that will present an early test of a Barack Obama administration.

His comments were seized upon by the Republican campaign yesterday to raise fresh doubts about the prepared-ness of Mr Obama to be commander-in-chief.

Speaking at a fundraiser in Seattle on Sunday night, Mr Biden said: “Mark my words, it will not be six months before the world tests Barack Obama like they did John Kennedy. The world is looking. We’re about to elect a brilliant 47-year-old senator president of the United States of America. Remember I said it standing here . . . we’re gonna have an international crisis, a generated crisis, to test the mettle of this guy.”

He cited Russia and the Middle East as possible places that may cause problems, as well as the border between Afghanistan and Pakistan – “crawling with al-Qaeda” – as being of particular concern.

Mr Obama would need help and support, Mr Biden suggested, “because it’s not gonna be apparent, initially, that we’re right.” He then spotted the media in the room, “I probably shouldn’t have said all this because it dawned on me that the press is here.”

John McCain told a rally in Belton, Missouri: “The next President won’t have time to get used to the office. We don’t want a President who invites testing from the world at a time when our economy is in crisis and Americans are already fighting in two wars.”

Both campaigns have been briefed by the Bush Administration on intelligence amid warnings that terrorists and rogue states will seek to exploit the three-month transition period after the election in November.

Fran Townsend, a former adviser to President Bush on homeland security, told a think-tank in Washington last week that history showed the early days of a new government could be the most vulnerable. She cited the first attempt to bomb the World Trade Centre at the start of the Clinton Administration in 1993; the Glasgow airport attacks a month after Gordon Brown became Prime Minister in 2007; and the September 11, 2001 terrorism during President Bush’s first year in office.

“I don’t feel confident that the campaigns are taking that seriously,” she said. “We would be remiss if we didn’t take notice.”

Democratic strategists dismissed suggestions that Mr Biden, the chairman of the Senate Committee on Foreign Relations who has a habit of making minor gaffes, had erred again by drawing attention to the threat. They emphasised that Colin Powell, the former Secretary of State and a Republican heavyweight, endorsed Mr Obama because he believed that the Democrat was better able to keep America safe.

Times Recommends

• Times Charity Appeal: SolarAid
• Is it Black Monday all over again for Dubai?
• Behind the scenes at the biggest deal