Bashir, who is wanted by the International Criminal Court (ICC) on charges of genocide and other crimes against humanity, submitted a visa request to the US, which is obliged to grant entry to foreign heads of state or representatives wishing to attend the General Assembly.
Although the US is not part of the ICC, it is part of the UN Security Council which voted to refer prosecutions for atrocities committed in Darfur to the ICC.
See the New York Times for full report.