Bryan Whitman, a Pentagon spokesman, said there was nothing unusual about the caveat because a defence secretary always has the authority to change or modify policy he has made.
"Any deviation from the policy would have to be approved," he told reporters. "The secretary can make an exception to any policy."
The new directive lays out broad policy governing interrogations of detainees in Department of Defence custody, but leaves the definition of "humane" to a separate, yet to be released directive that is still being debated within the administration.
A little noticed loophole in the directive, which was made public on Tuesday, gives the secretary of defence or his deputy authority to override the policy.
"Intelligence interrogations will be conducted in accordance with applicable law, this directive and implementing plans, policies, orders, directives, and doctrine developed by DoD components and approved by USD (I), unless otherwise authorised, in writing, by the secretary of defence or deputy secretary of defence," the directive states. "USD (I)" refers to the undersecretary of defence for intelligence.
The language in the directive echoes a struggle between the White House and members of Congress over a proposed amendment to the defense spending bill that would ban outright "cruel, inhuman and degrading treatment of prisoners in the detention of the US government".
Vice President Dick Cheney reportedly has pressed Senator John McCain, the amendment's sponsor, to exempt the CIA from the ban.
The White House denied on Tuesday it is seeking an "exemption for torture" for the CIA, despite President George Bush's threat to veto the legislation.
Meanwhile, US Attorney General Alberto Gonzales said in response to a question about a proposed law which would ban the torture of prisoners that existing US laws on torture are sufficient.
"There already existing domestic and international legal prohibitions binding upon the United States in regards to torture," Gonzales said following a speech at the University of Chicago.
"There already existing domestic and international legal prohibitions binding upon the United States in regards to torture"
US Attorney General
"We don't believe in torture. We don't condone torture. The president has made that very, very clear," he said.
"There are no circumstances in which torture we believe is permissible or should be used. Everyone understands that in the administration and everyone understands what the standard is, and if you don't meet that standard you're going to be investigated and you're going to be prosecuted."
Lobbying for exemption
The administration has come under pressure recently over reports of secret US prisons for terrorism suspects.
While he continues to state that the United States does not use torture, President George Bush has threatened to veto legislation banning "cruel, inhuman and degrading" interrogations of detainees by US forces and agents under any conditions.
The law has gained widespread bipartisan support, but the administration is lobbying to have the Central Intelligence Agency exempted from the ban.
On Tuesday the US Defense Department said it has issued a broad policy directive prohibiting physical or mental torture during military interrogations.