N'DJAMENA (Reuters) - Sudan moved to ease tensions with neighbouring Chad on Tuesday, sending a top minister to pledge assistance in fighting Sudanese Arab militias which Chad says have carried out attacks in recent days on its territory.

"We know that there are some illegal elements causing trouble in some places along the border," Sudan's Interior Minister Abdel Rahim Mohamed Hussein told reporters after meeting Chadian President Idriss Deby in the capital N'Djamena.

"But this cannot be allowed fundamentally to harm the solid relations between our two countries."

Chad's acting defence minister said on Sunday that so-called Janjaweed militias backed by Khartoum had carried out three attacks in less than a week on Chadian territory, and warned the Sudanese government to stop them or face the consequences.

An estimated 100,000 Sudanese refugees have fled over the border to Chad to escape fighting in Sudan's western Darfur region, where the horse- riding militias have been raping and pillaging while government planes bomb farming communities.

Medecins Sans Frontieres (MSF) said on Tuesday the situation for refugees was worsening, with the camps overcrowded, food and water scarce, and the risk of attacks from Sudan still high.

"The level of malnutrition is now climbing every week," Jean de Cambry, who recently returned from the border as the relief agency's emergency coordinator, said in a statement.

"In mid-April we were admitting three or four children in Iriba (near the Sudan border) with severe acute malnutrition each week. Now we admit nearly 25," he said.

Hussein said Khartoum recognised that the Arab militia were acting illegally and said they needed to be controlled.

Human rights groups and Western diplomats have previously been sceptical about Sudanese government assertions that it is not in league with the Janjaweed.

But Ahmad Allami, an adviser to Deby on international relations, said Chad was satisfied with Khartoum's stance.

"Relations between N'Djamena and Khartoum are no longer clouded," he told reporters.

Chad had brokered a truce between Khartoum and the two black African rebel groups it is fighting in Darfur to allow safe passage for humanitarian aid, but the fighting has resumed.

The United Nations and human rights groups have accused the Sudanese government and the Janjaweed of ethnic cleansing in Darfur and massive human rights violations which they say may constitute crimes against humanity.