Mr Straw should be armed with a smile and a very big stick

Jack Straw does not have an easy task as he flies into Sudan to view what the UN has called "the world's worst humanitarian crisis" in the remote western deserts of Darfur. For he must both smile and carry, if not a big stick, then certainly a clearly visible one.

He can expect the Sudanese to smile back, and to lie. Within a week the UN deadline requiring Khartoum to rein in the Janjaweed militia who have caused the crisis will expire. But they will not meet the deadline. Bad faith has been one of Khartoum's consistent hallmarks for decades. Which is why it has denied involvement in gross violations of human rights and at the same time tried to impede the access of the media, aid workers and UN atrocity investigators to Darfur. But their scorched earth tactics there are identical to those they have used for 20 years in the war on non-Arab rebels in southern Sudan.

The problem is that Khartoum is - under long pressure from the United States - close to a peace deal for the war-torn South. Mr Straw knows that this could be jeopardised by the rebel threat from Darfur, which is what has loosed again the ruthless instincts of a fearful Sudanese regime.

The West cannot afford to feed that fear with heavy-handed responses. In any case there is no backing for that in the Security Council where Russia, Algeria and Pakistan all diluted a recent proposal on sanctions. Talk of an oil embargo is unrealistic. China, which has provided Khartoum with three new arms factories in exchange for oil, will not even support an arms blockade.

None the less, the fact is that systematic attacks on villages in the South ceased when the US stepped up diplomatic pressure. The West must re-assert that pressure now. As Darfur's biggest cash donor, Britain carries some weight. Mr Straw should make clear not only that aid is at stake, but that the UK is prepared to give logistical support to the larger contingent of African Union troops proposed by the president of Nigeria. Most of all, he should make it clear that the West is unilaterally prepared to impose smart sanctions and travel restrictions which would hit the political elite in Khartoum. But he should smile as he says so.