By Jon Leyne BBC correspondent at the US State Department

There was an intriguing picture on Monday of the American Secretary of State, Colin Powell, and the Defence Secretary, Donald Rumsfeld, in a lively discussion outside the White House.

The zoom lens of a television camera caught the shot as the two men waited for President Bush and the Afghan leader, Hamid Karzai, to make an appearance in the Rose Garden.

Unfortunately, the camera's microphone did not record whether the two members of Mr Bush's cabinet were discussing their differences over how to treat Taleban and al-Qaeda prisoners held at Guantanamo Bay in Cuba.

Unusually for the Bush administration, it has been a very public argument, but it is not clear that it is over any real issue of substance.

The debate

President Bush confirmed that the matter was discussed at the National Security Council. This was where the debate had reached, he explained.

Secretary Rumsfeld toured Guantanamo and stressed detainees were not prisoners of war

"A couple of things we agree on. One, they will not be treated as prisoners of war. They're illegal combatants. Secondly, they will be treated humanely. And then... I will listen to all the legalisms and announce my decision when I make it," he said.

On the face of it, that doesn't leave much for the president to decide. The only argument is not whether the detainees are prisoners of war, but why they are not.

Under the Rumsfeld argument, the detainees should not be treated as prisoners of war, because the Geneva Convention Relative to the Treatment of Prisoners of War has no bearing on the case whatsoever.

Under the Powell argument, the Geneva Convention is applicable, however under the rules of that convention they are still not prisoners of war, but illegal combatants.

Important distinction

It might seem a rather arcane dispute.

Yet if Mr Powell wins, the detainees should win the right to a hearing before a "competent tribunal" to decide their status.

Until that happens, according to the letter of the convention, they should be treated as prisoners of war.

And that would severely restrict the right of the US authorities to question them - a big obstacle in the war on terror.

This is an argument that has caught Washington by surprise.

Despite the interest overseas, it is an issue the American domestic media were slow to pick up on.

Now it threatens one of the biggest setbacks to date in the war on terror. Almost for the first time since September the eleventh, the United States is on the defensive in the battle for international public opinion.