Britain on Monday warned newly-elected Iranian president Hassan Rowhani that he should be in no doubt about the West's resolve to prevent nuclear proliferation in the Middle East.
Foreign Secretary William Hague told parliament he hoped Tehran would now "engage seriously" with the West on its disputed nuclear programme.
Rowhani on Monday ruled out any halt to the nuclear activity that has drawn UN sanctions but said he hoped an early deal could be reached to allay the concerns of major powers.
Hague told lawmakers: "The government hopes that following Dr Rowhani's election, the Iranian government will take up the opportunity of a new relationship with the international community by making every effort to reach a negotiated settlement on the nuclear issue.
"If Iran is ready to make that choice, we are ready to respond in good faith."
Hague added: "Our commitment to seeking a peaceful, diplomatic settlement of this dispute is sincere.
"Iran should not doubt our resolve to prevent nuclear proliferation in the Middle East and to increase the pressure through international sanctions should its leaders choose not to take this path."
Hague said he hoped Iran would now "engage seriously" with the so-called E3 plus 3 negotiating with Tehran -- Britain, France and Germany, plus the United States, China and Russia.
On Syria, Hague said Iran was "a very long way" from playing a helpful part in resolving the conflict.
"A constructive role by Iran would be very welcome in Syria. There are opportunities for that as well," he said.
"Iran's policy at the moment is the exact opposite of that and there's an abundance of evidence of Iranian participation in the murder and torture and abuse committed by the (President Bashar al-) Assad regime.
"We are a very long way as things stand today from Iran instead playing a helpful, diplomatic, restraining role, highly desirable though that would be."
Hague indicated there were no immediate plans to reopen the British embassy in Tehran, after it was closed in 2011 following a mob attack that he said "could only have been state-sponsored in some way".
In the parliamentary exchanges, Jack Straw, Britain's foreign secretary from 2001 to 2006, said the international community should not expect too much, too soon of Iran's new leader.
Straw, who dealt with Rowhani as head of Iran's security council, nonetheless said he was "courteous, engaged and straightforward".
He said that, if given space, Iran could now become a positive force in respect of Syria, Lebanon, Afghanistan and Iraq.