KHARTOUM, June 19 (Reuters) - Sudanese President Omar Hassan al-Bashir has ordered "complete mobilisation" to disarm all illegal armed groups in the western region of Darfur, including the Arab militias who have been harassing African villagers.

The statement on Saturday comes a day after the United States threatened to impose sanctions on Sudanese officials as a way of intensifying pressure to help ease a humanitarian crisis in Darfur the United Nations says is the worst in the world.

The Sudanese presidency statement said all government agencies should mobilise "to control and pursue all outlaw groups, including rebels and Janjaweed..., disarm the outlaws and present them to justice and prevent any groups from crossing into neighbouring Chad".

The Janjaweed is the local word for the Arab militias whom the Darfur rebels blame for much of the conflict in the region.

The rebels say the government has backed the Janjaweed but the government has repeatedly denied that.

Sudanese Foreign Minister Mustafa Osman Ismail told reporters later the decision meant the government would deploy additional police and troops to maintain order in Darfur. He did not give any numbers.

Asked how the government could reconcile the decree with the ceasefire between the government and the rebels, he said the truce required the rebels to maintain current positions.

"Any group out of the demarcated territories -- it is the responsibility of the government to deal with them ... therefore the president's decree is not contradictory," he added.

International organisations have criticised the Sudanese government for failing to control the militias, who have driven hundreds of thousands of Africans out of their villages into camps for displaced persons or into exile in Chad.

The State Department is studying whether the militias are responsible for genocide in Sudan and if it can impose sanctions on individual officials, spokesman Adam Ereli told reporters.

The Sudanese announcement implied a tougher line against the Janjaweed and a more balanced policy towards the conflict.

Sudanese government officials have previously said it would be difficult to disarm the Arab militias as long as the two main rebel groups -- the Sudan Liberation Movement and the Justice and Equality Movement -- were active in the region.

It also answers Chadian complaints about Janjaweed incursions into Chadian territory. A source close to Chadian President Idriss Deby said on Friday the Chadian army killed 69 Janjaweed in a clash near the border on Thursday.

The presidential statement also said that the judicial authorities in Darfur should set up prosecution offices and courts to prosecute plunder gangs and criminals "without delay".

Sudanese police should deploy to protect villages and enable displaced people to come home, it added.

Analysts say that part of the problem in Darfur is that the central government in Khartoum, 1,000 km (600 miles) from the Chad border, does not have the resources to control the area.

More and more people are at risk as the seasonal rains cut off roads.

Alistair Dutton, an aid coordinator for the Catholic aid agency CAFOD in Darfur, said time was running out to bring in relief supplies and, without help, many people would die. "With continuing conflict, lack of food, water and sanitation and with extreme temperatures we fear high levels of death. The international aid effort has to be increased many times if we're to avert a humanitarian disaster," he said.

CAFOD has trebled its response and one of its partners, Norwegian Church Aid, is flying 41 tonnes of supplies to Nyala, capital of southern Darfur, on Sunday afternoon.