(CNN) -- The European Union's foreign policy chief said Sunday there is "no guarantee" a comprehensive nuclear agreement with Iran will work out.
Catherine Ashton spoke to reporters alongside Iranian Foreign Minister Mohammad Javad Zarif at a news conference in Tehran.
"I have been engaged in discussions with Iran for nearly four years, and I think this interim agreement is really, really important, but not as important as a comprehensive agreement that we are currently engaged in," she said, according to published remarks.
"Difficult, challenging, there is no guarantee it will succeed, but I think it's very important with the support of the people of Iran for the work that is going on by the minister and his team and with the support of the international community for my work that we should aim to try and succeed."
In November, Iran and six world powers forged an interim deal, under which Iran agreed to roll back parts of its nuclear program in return for relief from some sanctions. That agreement went into effect in January.
The challenge now is to reach a permanent deal acceptable to all sides.
The next round of talks between the so-called P5+1 -- the United States, China, Russia, Britain, France and Germany -- and Iran will start next week in Vienna.
Ashton's two-day visit to Tehran is the first official visit by an EU foreign policy chief since 2008, according to Iran's Press TV.
Besides Zarif, she met with President Hassan Rouhani and other top political and civil leaders.
"One of the things that's been very clear is the support that is given across the political spectrum for the work that is going on currently in Vienna to try and move forward on a comprehensive agreement.
"That does not mean that we'll get an agreement. It does not mean that people have committed to any possible outcome at this stage. But I have had a real sense that people are committed to wanting to see the talks happen and that, I think, is encouraging of itself," Ashton said.
The United States and its allies believe Iran is trying to develop a nuclear weapon, while Tehran has said its atomic efforts are peaceful.
CNN's Laura Smith-Spark contributed to this report.