US officials in Baghdad and Washington are expected to unveil a secret intelligence "dossier" this week detailing evidence of Iran's alleged complicity in attacks on American troops in Iraq. The move, uncomfortably echoing Downing Street's dossier debacle in the run-up to the 2003 Iraq invasion, is one more sign that the Bush administration is building a case for war.

Nicholas Burns, the senior US diplomat in charge of Iran policy, says Washington "is not looking for a fight" with Tehran. The official line is that Washington has made a conscious decision to "push back" against Iran on a range of fronts where the two countries' interests clash. Primarily that means Tehran's perceived meddling in Iraq, where its influence with the Shia-led government and Shia majority population appears to be increasing as Washington's weakens.

State department spokesman Sean McCormack claimed this week the administration has a body of evidence implicating Iran in sectarian attacks against Iraq's Sunni minority. "There is a high degree of confidence in the information that we already have and we are constantly accumulating more," he told the New York Times.

CIA and Pentagon officials are also touting intelligence that "Iranians are smuggling into Iraq sophisticated explosive devices, mortars, and detailed plans to wipe out Sunni Arab neighbourhoods," the paper said. Officials would make a "comprehensive case" this week. But President George Bush has already acted on information received. He confirmed yesterday that he has ordered US forces in effect to kill or capture Iranian "agents" targeting Americans in Iraq - as happened earlier this month when five Iranian officials were detained in Irbil.

Hassan Kazemi Qumi, Iran's ambassador to Iraq, ridiculed "sectarian maps" and evidence the US military said it had obtained during a raid on a Shia compound in Baghdad. He repeated Tehran's contention that Iranians were in Iraq to help with "security problems". Barham Saleh, Iraq's deputy prime minister, complains that the US and Iran are turning his country into a "zone of conflict and competition" and suggests they take their fight elsewhere.

But as was also the case in the days before Saddam Hussein fell, powerful external forces, ranging from exiled Iranian opposition groups to leading Israeli politicians, appear intent on stoking the fire - and winding up the White House.

"The al-Quds Force of Iran's Islamic Revolutionary Guards is stepping up terrorism and encouraging sectarian violence in Iraq," Alireza Jafarzadeh, a US-based Iranian dissident who has been linked to the Mujahedin-e-Khalq (MeK) resistance group, told the Washington Times this month. Mr Jafarzadeh is credited with revealing the existence of Iran's secret nuclear sites in Natanz and Arak in 2002.

"There is a sharp surge in Iran's sponsorship of terrorism and sectarian violence in the past few months," Mr Jafarzadeh told a conference organised by the Iran Policy Committee, a Washington lobby group pressing the state department to remove the MeK from its terrorist list.

Israel is also pushing the intelligence case while upping the ante, claiming to have knowledge that Tehran is within a year or two of acquiring basic nuclear weapons-making capability. In a BBC interview last week former prime minister Binyamin Netanyahu compared President Mahmoud Ahmadinejad's regime to Hitler's Nazis. Speaking in Davos the deputy prime minister, Shimon Peres, demanded immediate regime change or failing that, military intervention.

The US "push back" against Iran comprises many other elements beyond Iraq. Unconfirmed reports suggest Vice-President Dick Cheney has cut a deal with Saudi Arabia to keep oil production up even as prices fall, to undercut Iran's main source of foreign currency. Washington is pursuing expanding, non-UN global financial sanctions against Tehran; encouraging and arming a "new alignment" of Sunni Arab Gulf states; and highlighting Iran's role in "supporting terrorism" in Palestine, where it helps bankroll the Hamas government, and Lebanon, where it backs Hizbullah. The US is also deploying powerful naval forces in the Gulf that are of little help in Iraq but could more easily be used to mount air strikes on Iran.

Almost any one of these developments might produce a casus belli. And when taken together, despite official protestations, they seem to point in only one direction. The Bush administration, an American commentator suggested, is "once again spoiling for a fight".