By Paul Reynolds World Affairs correspondent BBC News website

The four-year conflict in Darfur has caused widespread misery

However, the attitude of Russia and China will be crucial. Both have so far been cautious about applying sanctions against Sudan.

At the moment, there is a limited UN ban which is supposed to prevent the supply of arms to any of the belligerent parties in Darfur, but it applies only in the territory of Darfur itself.

The wording, from resolution 1591, passed in March 2005, says that the ban on weapons should apply "to all the parties to the N'djamena Ceasefire Agreement and any other belligerents in the states of North Darfur, South Darfur and West Darfur."

This technically allows countries such as Russia and China to continue supplying weapons to the government of Sudan, as long as they are not used in Darfur.

Amnesty says that such weapons, including attack helicopters and jet fighters, are being used in Darfur and calls for the immediate suspension of the "transfer of arms to the parties to the conflict in Darfur".

It argues that Russia and China should be aware that their weapons are being deployed in Darfur and should therefore stop their sales.

Weapons sales

It notes that in 2005, the latest year for which the UN has figures, both Russia and China supplied Sudan with weapons.

China sent arms and parts worth $83 million and the value of Russia transfers was more than $34 million.

It claims that the Chinese company AviChina Industry and Technology "recently delivered six K-8 military training/attack aircraft to the Sudanese Air Force and a further six will follow soon."

This would not be illegal under the UN embargo, but it would be illegal for the government of Sudan to use them in Darfur.

China and Russia

China has recently been willing to speak more strongly to Sudan, to get it to accept a full UN peacekeeping force in Darfur. Sudan has agreed only to a partial force so far and even this has not been deployed.

Amnesty says a white Antonov 26 was spotted at Nyala in March

However, China has significant oil interests in Sudan and, according to the Council on Foreign relations in New York, takes 64% of Sudan's oil exports. It is therefore unwilling to push Sudan too far.

Russia also has oil investments in Sudan, but it has its own oil and does not have the desperate Chinese need to find it around the world. It has also traditionally been a major arms supplier to Sudan.

However, oil does not explain Russia's caution, which is probably more associated with Russia's current determination to distance itself from what it sees as overly influential American and other Western policies generally.

The Russian Foreign Ministry, responding to the Amnesty report, was careful in its wording, which showed that it is fully aware that the UN embargo applies only in Darfur.

"None of our arms are being supplied to Darfur," a statement said.

Chinese Foreign Ministry spokeswoman Jiang Yu made a similar point, saying they did not sell arms to regions under a UN embargo.

UN moves

Discussions are currently going on at the Security Council about extending the UN embargo to apply it to the government of Sudan as a whole.

The European Union already imposes such an embargo and the United States has a similar one. But the UN policy is that the Sudanese government is not supposed to let its weapons be used in Darfur but the Amnesty report charges that it is doing precisely that.

The Amnesty report says that the UN arms embargo is "somewhat vaguely formulated (and lacks) a strong UN monitoring, verification and public reporting mechanism."

The British government is supporting an extension of the UN arms embargo to bring it into line with one imposed by the European Union. This applies to Sudan and therefore its government as a whole. The United States also imposes similar sanctions.

However, a Foreign Office spokeswoman in London said: "It is a long and difficult process."