Ashton said Wednesday that it was a "very intensive and I think a very important meeting," according to The Associated Press.
"We have reached a serious stage in the talks," Zarif told reporters.
The two sides have agreed to meet again in Geneva on Nov. 7-8.
This week’s talks were the first since Iranian President Hassan Rouhani was elected in June, which sparked new hopes of an agreement could be reached.
Iran is hoping to get painful sanctions from the United States and Europe over its nuclear program lifted.
The AP reported that a member of one of the delegations at the talks said the new Iranian plan offered reductions in the levels of uranium enrichment as well as the number of Iran’s centrifuges.
Iran’s semi-official Fars News Agency said that Iran’s proposal would occur in three stages.
The Obama administration has said it is willing to quickly lift sanctions should Iran prove that it is willing to follow through, but U.S. officials have lowered expectations of a quick “breakthrough.”
“We certainly want to make clear that no one — despite the positive signs that we've seen — no one should expect a breakthrough overnight,” White House press secretary Jay Carney said Tuesday. “We will be looking for specific steps that address core issues, such as the pace and scope of its enrichment program, the transparency of its overall nuclear program and its stockpiles of enrichment.”
Congress has pushed tougher sanctions against Iran’s oil exports this year, which have passed the House already. Senate Banking Committee leaders said they weren’t going to stop pursuing them because of Rouhani’s election, but a group of 10 senators said they would consider suspending new sanctions should Iran suspend its uranium enrichment.