The decision was taken by the US Secretary of State, Warren Christopher. For months, US officials have been giving off-the-record briefings in which they have pointed the finger at Sudan for harbouring what they called terrorist groups, including the Palestinian Abu Nidal organisation, the Lebanese Hizbollah, and the Palestinian Islamic Resistance Movement, Hamas.
They have expressed concern at what they have identified as growing co-operation between Iran and Sudan, and suggested that Iranians have been training Sudanese and others in training camps in Sudan.
However, the same officials also acknowledge that they have no hard evidence that a single terrorist act has been launched by these groups from Sudan. The last time that Abu Nidal struck in Khartoum - when bomb attacks on the Acropole hotel killed British aid workers - took place during the democratically elected government of Sadiq al-Mahdi. Since the military coup of General Hassan al- Bashir introduced a more Islamic regime, Abu Nidal's men have been monitored coming and going, but not perpetrating any action.
State Department officials are at pains to emphasise that the decision is entirely unrelated to the investigations into the 26 February bombing of the World Trade Center in New York. They state that the evidence of the Sudanese government being implicated is at best flimsy.
'The key thing is that (the decision) is not related to the investigation in New York or the bombing plots . . . (but) to Sudanese sponsorship of terrorist organisations and its acquiescence in terrorist training within Sudan itself,' one official stated.
In the 1992 report Patterns of Global Terrorism, published in April 1993 by the State Department, a photograph of the leader of Sudan's National Islamic Front, Hassan al-Turabi, was displayed in a prominent place. Yet the report conceded that 'there is no evidence that the government of Sudan conducted or sponsored a specific terrorist attack in the past year'.
Sudan, however, has an open-door policy towards any Arab who seeks refuge, especially those not welcome in their home countries. Many Arab Islamic militants, mainly Egyptians and Palestinians, who had fought alongside Afghan mujahedin, came to Khartoum in May after being expelled from Pakistan.
Why then did Mr Christopher act now? The key may lie in his past. He was deputy secretary of state in the administration of Jimmy Carter, whose key foreign policy achievement was the Camp David accords between Egypt and Israel. For Mr Carter, Egypt was the cornerstone of US policy in the Middle East. Mr Christopher feels Egypt was neglected during the Bush presidency.
The Egyptians have been putting increasing pressure on the US government to act against Iran and Sudan, the two countries it has accused - with little supporting evidence - of fomenting Islamic extremism against the state. Furthermore, the Egyptians are very displeased with the US over the handling of the blind sheikh, Omar Abdel-Rahman, spiritual mentor of Islamic militants in both New Jersey and Egypt.
By putting Sudan on the list of states said to be sponsoring international terrorism, Mr Christopher has not greatly changed US relations with Khartoum. But he will have given a sop to his Egyptian friends.