Powell vows proof on Iraq weapons
Baghdad has already dismissed the evidence.
"They won't be really proof, they will be fabricated space and aerial photos," Hussam Mohammad Amin, head of Iraq's National Monitoring Directorate, said on Sunday.
In a Wall Street Journal opinion piece two days before he speaks to the UN Security Council, Mr Powell wrote that although there was still no "smoking gun", the world must recognise Iraq had flouted the will of the international community.
Mr Powell was backed up by British Prime Minister Tony Blair, Washington's main ally in its drive to force Baghdad to give up banned weapons peacefully or by force.
Mr Blair said there was "unmistakable evidence" that Iraq was withholding cooperation from the inspectors.
Striking a conciliatory but firm note towards other European allies, many of them sceptical of US intentions, Mr Powell promised Washington would try to bridge differences and fully consult its partners before any decision to go to war.
A peaceful outcome was still possible if Iraq's President Saddam Hussein switched course and cooperated with UN experts hunting for Baghdad's weapons of mass destruction.
But Washington "will not shrink from war" if necessary, he wrote.
Mr Powell did not say what evidence he would hand the Security Council tomorrow.
One of his aides told an Egyptian daily it would not have the detail of evidence used in a law court but would convince "any sensible person" Iraq was hiding something.
Richard Haass, US State Department director of policy planning, told al-Ahram in an interview run in Arabic: "I don't want to raise expectations.
We will not present pictures of 30,000 stockpiled warheads which can each carry chemical weapons, if that is what you mean by evidence."
The two top UN disarmament officials, Hans Blix and Mohamed El-Baradei, are expected to go to Baghdad on Saturday despite a letter they wrote that appeared to put conditions on the trip.
Iraq has rejected any conditions for the visit, just days before another key report by the inspectors to the Security Council on February 14 possibly the last before Washington finally decides on an attack.