Government troops fighting rebels in Darfur have been backed by militias -- known as Janjaweed -- that have been accused of murder, torture, widespread rape and other human rights abuses against the civilian population.
Zoellick, who was speaking after talks with visiting African Union (AU) peace and security commissioner Said Djinnit, also called for the deployment of additional AU troops to help secure peace in Darfur.
"The message we have is that where we have AU forces conflict doesn't occur ... and that's one reason why a key element of the strategy is to expand the AU force presence," he said.
The AU team is monitoring a shaky ceasefire between Khartoum and Darfur's ethnic minority rebels, and last week international donors pledged almost 292 million dollars in further aid for the AU mission.
But the body is looking for more than 460 million dollars in cash, military equipment and logistical support to boost its current 2,700-strong truce monitoring operation to more than 7,700 by September.
Zoellick also suggested that AU troops be deployed in camps housing more than two million people that were displaced by the conflict in Darfur, saying AU patrols had brought down the number of reported rapes and attacks on camp residents.
"The key is to get the police forces operating within the camps and so that is being expanded and we talked about 30 additional posts in the camps," he said.
Zoellick visited Kutum town in north Darfur state and walked through Kassab refugee camp, which houses about 20,000 people who fled their homes. Hundreds of children chanting "Welcome, welcome" crowded around him.
He met tribal leaders who told him they wanted more police and AU forces to protect their people.
"We will only go home when we are accompanied by international or American troops," tribal leader Adam Ahmed Ajain said.
Zoellick assured them that more AU forces would be coming.
Meanwhile, the International Committee of the Red Cross said it decided to boost efforts to bring much-needed food to residents in remote areas in Darfur where "food supplies continue to run critically low."
"The ICRC will now undertake an intensive two-month airlift operation out of Khartoum," a statement said. From Saturday an aircraft will carry out two flights a day, six days a week, splitting its payload between Nyala and El Fasher. The food, mainly sorghum, lentils and cooking oil, will then be trucked to rural areas.
The ICRC said the operation will cost about 2.2 million dollars.
Humanitarian officials have warned that the situation in Darfur was growing more desperate because of shortfalls in funding, drought, famine and the long-term effects of conflict.
Besides leaving 2.4 million people homeless, the conflict has also killed between 180,000 and 300,000 people, mostly ethnic African civilians.
A United Nations resolution last March asked that alleged perpetrators of war crimes in Darfur, among them government officials, be tried by the International Court of Justice. Khartoum squarely opposes that prospect.
Both Zoellick and Djinnit underscored the need for a political solution to the conflict.
"It's not enough to provide humanitarian support and food and security here. We need to press forward the peaceful reconciliation process," the US diplomat said.
Talks between rebels and Khartoum are due to resume in the Nigerian capital of Abuja on June 10, after a six-month hiatus.
Zoellick, who was last in Darfur in April, is due to wind up his visit in Khartoum for talks with Sudanese Vice President Ali Osman Taha and United Nations special envoy Jan Pronk.
Djinnit is part of a delegation of the pan-African body visiting Darfur to assess the humanitarian situation there.
The visits follow a tour by UN chief Kofi Annan who warned that the world was running "a race against time" to solve the conflict.