Ismail had a "long" meeting on Thursday with Zoellick, who told him that "we are looking to results in Sudan and that actions speak more loudly than words," deputy State Department spokesman Adam Ereli said.
Ismail's visit is seen as significant, especially after Washington had accused his government of fuelling "genocide" in Sudan's Darfur region.
After the 90-minute meeting, Ismail was scheduled to meet Secretary of State Condoleezza Rice on Friday.
Zoellick "stressed the need for the government of Sudan to take action and all sides to take action to halt the violence," Ereli said, adding that the peace process in Abuja, was among key topics.
African Union (AU) mediators and parties in the Darfur conflict have set up a working committee specifically to get past differences that have stalled full peace talks in Nigerian capital Abuja for almost two weeks.
The fifth round of the AU-mediated talks to end the civil war has been deadlocked over rebel opposition to a mediation role for neighbouring Chad, thus halting the adoption of a key peace declaration by warring parties.
Robert Zoellick: Actions speak
more loudly than words
Zoellick and Ismail discussed the "importance of continued support" for humanitarian aid and for the expansion of an African Union peacekeeping force and the importance of removing any impediments to the expansion of that force.
The AU has 3320 troops in Darfur and the number is set to double by the end of September, before ultimately reaching 12,000 soldiers.
The conflict is believed to have left between 180,000 and 300,000 people dead and displaced some 2.4 million from their homes, with 200,000 fleeing into neighbouring Chad.
A ceasefire, concluded in April last year, has never been respected.
The two men also discussed the North-South issue in Sudan and steps being taken to finalise an interim constitution and to set up a national unity government.
An agreement ending nearly two decades of conflict in the south that left more than 1.5 million people dead and displaced some four million was signed recently and called for the passing of a constitution to govern the country during a six-year interim period.
Zoellick, who has made two visits to Sudan in recent months, talked about the possibility of his going to Khartoum for the 9 July inauguration of the national unity government.