Paul Reynolds BBC News Online world affairs correspondent

The United States and Britain have piled more pressure on Saddam Hussein, accusing him of "omissions" in the Iraqi weapons declaration but at the same time giving him another chance to make amends.

Jack Straw: building up pressure on Iraq

The UK Foreign Secretary Jack Straw's statement that Iraq had not made the "full and complete declaration" required by the UN and that the dossier was an "obvious falsehood" is expected to be followed by a similar one from US President George W Bush.

British officials made clear that Iraq still had time to comply. It would have to explain, they said, what happened to chemical and biological materials which the UN said had not been accounted for when it left Iraq in 1998.

The officials denied that the "zero tolerance" promised by President Bush had been relaxed. They said that resolution 1441 required both a failure to co-operate as well as a false statement before Iraq could be declared in material breach of the resolution.

In the British view, the false statement has been made. If Iraq fails to correct it, that could be classed as non-co-operation and would be taken into account in a final assessment of Iraqi compliance.

Intensive inspections

The accusation sets the stage for a much more intensive inspection phase lasting several weeks in which the idea would be to catch Iraq red-handed or "in flagrante delicto" as one official quoted by the Los Angeles Times put it.

The idea would be to get firm evidence in order to strengthen the case for possible war.

At the moment, the US asessment is that, as the Secretary of State Colin Powell described it, "there are problems" with the Iraqi declaration.

But world opinion and opinion within the United States might not be convinced by a simple accusation. So the hunt will be on for proof.

And the pressure will also be on the weapons inspectors, with Washington calling for Iraqi scientists who hold the secrets to its weapons programmes to be interviewed, abroad if necessary, as is allowed for in the resolution.

The "problems" over the declaration relate in part to material unaccounted for when the weapons inspectors left Iraq in 1998.

Explaining what happened to it was the first test laid down by the US and Britain for the truthfulness of the Iraqi inventory.

When Iraq handed over the declaration on 7 December, in fact, it said - worryingly - that there was nothing new to say about this material.

Missing material

It was made up of 300 tonnes of chemical warfare agents, including 1.5 tons of VX, 3000 tons of chemical precursors, growth medium for 20,000 litres of biological warfare agents and tens of thousands of shells and bombs for use in chemical and biological war.

Powell said US criticism over the dossier was "well-founded"

Iraq said at the time that all this had been destroyed but the UN was not convinced.

The next stage is for the head of the inspections organisation (Unmovic), Dr Hans Blix, to give his own preliminary view to the Security Council in New York on Thursday 19 December.

A key date after that is 27 January.

That is the deadline for Unmovic and the International Atomic Energy Agency (IAEA) to give a more considered view on Iraqi compliance.

Theirs will not determine the American (or the British) attitude.

But it will be important and unless they say that Iraq is not co-operating, other members of the Security Council would be unlikely to authorise military action against Iraq.

'Grey area'

Before there can be any military action either by the US or the Security Council as a whole, Iraq has to be in "material breach" of the resolution.

Iraq says the US looks for any reason to attack

If Iraq is reported to the Council for failing in some way, a meeting of the Council has to be convened immediately.

But the resolution does not commit a meeting to do anything.

The wording does remind Iraq that it has been warned before of "serious consequences" if it does not comply with UN resolutions.

Everyone knows, of course, what "serious consequences" could mean.

If the US, supported perhaps by Britain, decided to take unilateral action, it would probably wait until the Council has been unable to reach agreement first.

All this would probably take some weeks. There are many "ifs" in this story.