The hawkish Deputy Defence Secretary Paul Wolfowitz will spend two days holding talks in London and at the Nato headquarters in Brussels, before travelling on to Turkey - whose airbases could play a key role in any attack against Iraq.
He will be accompanied by the State Department's Under Secretary for Political Affairs, Marc Grossman, who will go on to Cyprus, Greece, Germany, the Netherlands, France and Portugal after Mr Wolfowitz heads back to Washington on Tuesday.
Correspondents say the officials will be following up American requests to governments for military contributions, and seeking to build a more solid political coalition against Iraq.
The US has threatened to use force to disarm Iraq, which it alleges is in possession of weapons of mass destruction.
Freeze
On Saturday, UN weapons inspectors in Iraq mounted further searches for hidden weapons, including a visit to the "Mother of All Battles" military complex, just south of Baghdad.
Inspection teams also spent several hours at another complex nearby and went to a facility at Balad, some 90 kilometres (56 miles) north of the capital, where the Iraqis hold chemical warfare training for military personnel.
The arrival of the inspectors at Balad seemed to take the Iraqis slightly more by surprise than in previous days, according to the BBC's Kim Ghattas in Baghdad.
Soldiers scrambled and Iraqi officers accompanying the inspectors screamed orders to freeze all movement on the site.
The inspectors resumed their work on Wednesday after a four-year absence, operating under a new UN Security Council mandate that gives Iraq a "final opportunity" to shut down its chemical, biological and nuclear weapons programmes or face "serious consequences".
The UN teams have so far visited sites with well-known involvement in Iraq's past weapons programmes, and places where equipment had been disabled or destroyed after UN inspections in the 1990s.
So far, access has been granted swiftly and all questions answered.
But our correspondent says the real test will come when the inspectors arrive at a facility which the Iraqis are not aware is on the UN's list for inspection.