The US Pentagon has set up a special planning group to co-ordinate possible attacks on Iran, investigative journalist Seymour Hersh has reported.
The US denies it is preparing to attack Iran
In the weekly New Yorker magazine, Hersh said the group would plan raids to be carried out within 24 hours of a presidential order for military action.
A Pentagon spokesman said he knew of no such group and denied that the US was planning war with Iran.
Any suggestion of this was "wrong, misleading and mischievous", he said.
The article, quoting unnamed sources, added that covert intelligence-gathering inside Iran had been stepped up in recent months, with special operations groups entering from Iraq to pursue Iranians suspected of working against US interests.
The operations were co-ordinated by Vice-President Dick Cheney and relied heavily on support from Saudi Arabia, the sources said.
But the fact that Sunni groups were receiving US backing meant that militants hostile to US interests were benefiting from the policy, they added.
Quoting a senior former intelligence official, Hersh wrote that a group had been set up within the Joint Chiefs of Staff to develop a contingency bombing plan which could be implemented within 24 hours.
"The Pentagon is continuing intensive planning for a possible bombing attack on Iran, a process that began last year, at the direction of the president," he wrote.
The group had recently acquired a new assignment, he said, to identify targets in Iran that could be supplying or aiding militants in Iraq.
Previously the focus had been on the destruction of Iran's nuclear facilities and regime change, the reporter added.
But Pentagon spokesman Bryan Whitman told the Associated Press news agency he did not know of the existence of the group, adding that the US was not planning to go to war with Iran.
"To suggest anything to the contrary is simply wrong, misleading and mischievous," he said.
Republican Congressman Duncan Hunter also played down the reports.
"We have contingency plans around the world," he said, quoted by AP.
"We had contingency plans with the Soviet Union, and we had specific targets. That didn't mean that we were planning to strike the Soviet Union."