A senior western diplomat cautioned on Thursday that any breakthrough in diplomacy over Iran's nuclear program was “not close", according to the Reuters news agency.
The comments come after two days of nuclear talks with Iran in Geneva, which Western negotiators have described as the most detailed and serious to date.
During the two-day session, Iran presented what it described as a breakthrough proposal that would include snap inspections of its atomic sites.
The proposal was described by the White House as "useful". White House spokesman Jay Carney said it showed a "level of seriousness and substance that we have not seen before."
Despite the improved atmosphere, however, diplomats said on Thursday that major differences remained between western governments, which suspect Iran's nuclear work has covert military goals, and Tehran, which denies that and demands the lifting of crippling economic sanctions.
In Brussels, a senior diplomat told Reuters the talks in Geneva - the first such meeting since President Hassan Rouhani took office in Iran in August - had left negotiators "more reassured than we were before".
"We learned more about their program and their concerns," the diplomat said, speaking on condition of anonymity.
"However, it doesn't mean we are close to a solution and that we will have an agreement next month," he stressed.
Rouhani, who assumed office two months ago succeeding conservative Mahmoud Ahmadinejad and who is described as a “moderate”, has pledged transparency on Iran's controversial nuclear program in an effort to get crippling UN sanctions lifted.
After presenting Iran’s proposal to break the deadlock with world powers over its nuclear program, senior Iranian negotiator Abbas Araqchi and his U.S. counterpart Wendy Sherman met one-on-one.
That meeting marked the first direct nuclear talks between the Islamic republic and Washington since 2009.
Peaceful overtures made by Iran's new President Hassan Rouhani have led to direct talks with the West, although Israel has yet to be convinced that Iran has dropped its ambitions to acquire a nuclear weapon. As recently as Monday, Iran's supreme leader, Ayatollah Ali Khameini, continued to fuel doubts as he appealed to Muslims around the world to unite against Israel.
Israel has said that anything short of a cessation of uranium enrichment would not safeguard the Middle East and the world from the reality of an Iran with a nuclear weapon.
A senior Israeli official in Prime Minister Binyamin Netanyahu’s office on Wednesday issued a call on Western powers to keep up economic sanctions until Tehran gives up its nuclear program.
"Iran should be tested by its actions, not its proposals," the official, who was speaking on condition of anonymity, said.