America "needs the United Nations" to provide legitimacy for a war in Iraq, Clare Short warned on Sunday.

The cabinet minister said war without UN backing would lead to "more conflagration and trouble in the Middle East and a much more difficult military operation".

The international development secretary also warned of the potential human consequences of a conflict.

She said it was "regrettable" that it had taken so long for the "full consideration of all the humanitarian aspects has come to the fore".

Widely tipped to leave the cabinet in the event of war without UN support, Short would only say that "if we go through the UN and do the right things, I will fully support them".

In an interview for GMTV Sunday Programme, the left-winger praised last weekend's Stop the War demonstrators, saying "thank heavens all that happened".

Short said that the protests had boosted the argument for a second resolution.

"I actually think all the demonstrators helped to strengthen our government in that role because everyone can see that Tony Blair needs the UN to have his country with him," she said.

"I think there's two futures: One, where the UN leads, where the UN's authority is re-enforced, where justice is what the international community stands for, and the people of the Middle East see, at last, a Palestinian state coming forward and a just and decent settlement for the people of Iraq.

"Or, it's American power and division and the risks of much more conflagration and trouble in the Middle East and much more difficult military operation, and the risks of division and endless fighting within Iraq."

She emphasised the need for the United States to see that it needs the UN.

The prime minister had to continue to "get the US to see that it needs the UN, it needs the rest of the international community". This would "halt these divisions that have been breaking out, that don't help anybody", she said.

Short added that she was "increasingly hopeful" the security council would find enough common ground with the US and Britain to find a peaceful path or prosecute a war together.

But she admitted that she had had "sleepless nights" over Iraq and that Labour Party members were right to be sceptical about the government's actions.

"I've had my sleepless nights, but I think you should," she said.

"And I'm determined to do what I can with the leverage I have and to stand up for what's right, come-what-may, for me, and I understand where the country is and I understand where the Labour Party is...[that] we could contemplate the use of force to back the UN, I think the party's completely in tune with the overwhelming bulk of British public opinion, and actually public opinion across the world.

Warning of the humanitarian dangers of a war against Iraq, the minister responsible for delivering international aid argued that the humanitarian aspect is considered in every element of policy.

"In the case of Iraq, it's only latterly that the full consideration of all the humanitarian aspects has come to the fore, and I'm glad that's now there, but it has been late and it is regrettable," she said.

Any military action must be designed to minimise humanitarian consequences she insisted.

"You need military action that's very carefully designed to minimise any harm to people, and you need very quick action to keep people fair, to start to rebuild institutions," she said.

And she stressed that any post-Saddam government had to be "legitimate".

"You need legitimacy so people know they're not being oppressed or invaded, that their country's going to belong to them," she said.

"This is a country that has been brutally oppressed where people are in very, very poor shape, how are you going to keep order, keep people fed, reassure them that no-one's trying to take their country or oppress them?"