By Phil Mercer BBC News, Sydney

Mr Howard will end his tour in Dublin

John Howard has been a loyal ally of US President George W Bush and will be honoured with an official state dinner at the White House.

The veteran Australian leader will also visit Canada and Ireland.

Mr Howard has brushed aside jibes from opposition politicians that his two-week trip is merely a holiday.

He said there was some very serious business to attend to.

Before Mr Howard left, he said the showdown with Tehran would be a severe test for the United Nations.

He insisted that he was in favour of a diplomatic solution to the crisis.

China is also likely to be on the talks agenda.

Loyal ally

In Washington, the Australian leader will be feted as an international statesman.

He is held in very high regard for his unequivocal support for US foreign policy.

This week Canberra said it would double its troop numbers in Afghanistan. It also has several hundred soldiers in Iraq.

The US is Australia's closest and most powerful ally. Mr Howard will hold talks with Mr Bush and key members of his administration.

He will then meet his Canadian counterpart, Stephen Harper, in Ottawa.

A flight across the Atlantic will then take Mr Howard to Dublin.