The Bush administration warned Saudi Arabia, until this year one of its closest allies, to stop undermining the Iraqi prime minister, Nouri al-Maliki.

The US secretary of state, Condoleezza Rice, and the defence secretary, Robert Gates, are scheduled to visit Jeddah next week.

Reflecting the deteriorating relationship, the US made public claims that the Saudis have been distributing fake documents lying about Mr Maliki.

The Bush administration, as well as the British government, is telling the Saudis, so far without success, that establishing a stable government in Iraq is in their interest and that they stand to suffer if it collapses.

Relations have been strained since King Abdullah unexpectedly criticised the US, describing the Iraq invasion as "an illegal foreign occupation".

That was the first sign of a rift between the two, who have enjoyed a solid relationship for decades, based on Saudi's vast oil reserves.

At a briefing, the state department spokesman, Sean McCormack, did not refer directly to US frustration with Saudi, beyond saying that Ms Rice and Mr Gates, on their trip to the region, "will be wanting more active, positive support for Iraq and the Iraqi people".

The British government, which retains a close relationship with the Saudis, shares many of the US's concerns about Riyadh's role in Iraq but, unlike Washington, is unwilling to go public.

A Foreign Office spokesman said yesterday: "We have always encouraged the Saudis to participate in the political process in Iraq. Saudi Arabia has a crucial role to play and the Saudis recognise the success of the whole project for the region's stability."

The US claims the Saudi royal family is offering financial support to coreligionist Sunni groups in Iraq opposed to Mr Maliki's Shia-led government.

In a graphic example of the tension, Zalmay Khalilzad, until recently the US ambassador to Baghdad, protested to the Saudis over fake documents distributed in Baghdad which claimed Mr Maliki was an Iranian agent and had tipped off the radical Shia cleric, Moqtada al-Sadr, about a US crackdown on his Madhi army militia.

Mr Khalilzad, who is now US ambassador to the UN, wrote in the New York Times last week: "Several of Iraq's neighbours - not only Syria and Iran but also some friends of the United States - are pursuing destabilising policies."

The Bush administration is also expressing its unhappiness with the Saudis for failing to stem the flow of Saudi jihadists across its border to fight in Iraq, often as suicide bombers. The US estimates that about 40% of the 60 to 80 foreign fighters entering Iraq each month are from Saudi Arabia.

The administration, like Britain, is still dependent on oil from Saudi and until now has been reluctant to go public about the increasing differences with the kingdom. Other causes of tension include Saudi support for Hamas in Gaza and lack of support for a US Israel-Palestinian peace plan.