CAIRO (Reuters) -Sudan's army said on Friday it had killed Ali Yagoub Gibril, a senior commander for the paramilitary Rapid Support Forces (RSF) who was under U.S. sanctions, during a battle in the besieged north Darfur city of al-Fashir.

There was no immediate comment from the RSF.

Gibril was a leading commander for the RSF in al-Fashir, the last major city in the Darfur region of Sudan that the paramilitary force does not control.

The army said in a statement Yacoub was killed as an RSF attack was thwarted early on Friday by its troops and allied "joint forces" fighting alongside it - a reference to non-Arab former rebel groups from Darfur that are aligned with the army.

The RSF has been besieging al-Fashir, a city of 1.8 million people, for weeks and top U.N. officials have warned that the worsening conflict there could trigger widespread intercommunal violence.

The U.N. Security Council called on Thursday for a halt to the siege.

War between the army and the RSF erupted over conditions for a transition to democracy in mid-April last year in the capital Khartoum, soon spreading to other parts of the country.

The conflict has led to the world's largest displacement crisis, renewed ethnic violence in Darfur blamed on the RSF and its allies, and a sharp increase in extreme hunger.

(Reporting by Adam Makary, Tala Ramadan and Nafisa Eltahir; writing by Aidan Lewis; editing by Tomasz Janowski and Mark Heinrich)