Sudan said yesterday it was doing all it could to stem violence in Darfur as US Deputy Secretary of State Robert Zoellick visited Khartoum to pile on pressure backed by huge aid pledges. "We are working diligently to stop the violence, not only to stop the violence but to resolve the conflict through political negotiations and get Darfur back to normalcy," First Vice President Ali Osman Mohamed Taha told reporters.He later held talks with Zoellick, who is on a two-day visit to Africa's largest country to keep up pressure on Sudan to implement a North-South peace deal and end killings in Darfur. Taha often has made such statements about Darfur but reports from regional aid workers say Sudan has failed to deliver. Donors exceeded Sudan's aid requests Tuesday by pledging $4.5 billion at an international conference in Oslo to help the south recover from Africa's longest civil war. The United States, with a pledge of $1.7 billion, is the largest donor and Zoellick has made clear aid could be halted if Khartoum does not act on Darfur, which he said cast a shadow over the country's future.