Dr Hans Blix said there had been an “erosion” of the inspection regime and pressure needed to be put on Iraq to comply with the UN-mandated checks.
He was speaking after a meeting with US Secretary of State Colin Powell and Mohammed El-Baradei, head of the International Atomic Energy Agency which would be responsible for checking any nuclear programme in Iraq.
Mr Powell said the US favoured a single, strong new UN resolution demanding action from Iraq but acknowledged there were other views in the Security Council, which have been hardening against the US.
The Americans have circulated a controversial draft resolution which carries a clear threat of military intervention if Baghdad fails to comply with the mission of the weapons inspectors.
Dr Blix had wanted to return to Baghdad by the middle of this month, but acknowledged it would be awkward if his team received new instructions from the Security Council once they were in Iraq.
Instead, he said, it would be helpful to have new, more stringent rules for the Iraqis to follow before they left.
“The Security Council resolution that is now being discussed is one that I think we would welcome,” he said.
“It’s clear that there has to be constant pressure to get the Iraqis to comply,” he added.
Mr Powell said he was optimistic that an agreement would be reached soon among the 15 members of the UN Security Council.
“There has been, as Dr Blix said, an erosion of the inspection regime… and we have to fix that,” he said.
But opposition to the US line on Iraq has been hardening at the United Nations, with Russia and France rejecting the tough draft resolution on weapons inspections.
Russia’s President Vladimir Putin said on Friday that UN weapons inspectors had to go back to Iraq “as soon as possible”.
A spokesman for the French Foreign Ministry later said there was “no legal precondition for a new resolution” on weapons inspections in Iraq.
And Turkey, a key US military ally bordering Iraq, said any US attack on Iraq must have international backing.
A senior US official told the Reuters news agency that Washington was working with Britain and France to reach a compromise.
Britain, a permanent member of the UN Security Council, has already said it will support the US, but any resolution could be brought down by a veto from one of the other permanent members – France, Russia or China.
The French have been pushing a two-phase set of resolutions – the first outlining the inspectors’ mission and the second, only if needed, detailing action to be taken against Iraq.
The US said again on Friday that Iraq had illegal programmes to develop weapons of mass destruction and was moving to conceal them from any future inspections.
The US and Britain do not want inspectors from the UN Monitoring, Verification and Inspection Commission (Unmovic) on the ground in Iraq until a new UN mandate has been agreed.
President Bush is to deliver a televised speech on Iraq on Monday.
Meanwhile, bombing in Iraq’s no-fly zones has continued, with a strike on an air defence command centre on Thursday in retaliation for an attempt by the Iraqi military to shoot down US and UK planes dropping leaflets warning Iraqi gunners not to fire on planes patrolling the zones.