Kofi Annan, the joint United Nations-Arab League envoy on Syria, has welcomed Iranian support for his efforts to secure peace in the country, telling Tehran that it can be "part of the solution".
Annan was speaking in Tehran on Wednesday following talks with Ali Akbar Salehi, the Iranian foreign minister.
But while endorsing Annan's peace plan, which calls for a ceasefire by Thursday, Salehi said Syria's government needed to be given time to implement reforms.
Tehran is considered a key regional ally of Syrian President Bashar al-Assad, who faces growing international pressure over the crackdown by security forces that has seen cities shelled and thousands of people killed.
Annan stressed again the urgency of finding a way to end the killing and to provide humanitarian assistance to those in need, before getting all parties to the table.
"The political process must be Syrian-led and respect the aspirations of the Syrian people," Annan said. "What is important is that governments in the region and beyond work with Syria to resolve the crisis.
"The geopolitical position of Syria is such that any miscalculation can have unimaginable consequences."
Regarding a ceasefire agreement which requires Syrian government forces to halt operations by April 12, Annan said he had received assurances that the deadline would be honoured.
"If everyone respects it I think by six in the morning on Thursday we shall see improved conditions on the ground," Annan said.
Answering a question whether he supported calls by some countries, such as Saudi Arabia and Qatar, to arm the Syrian opposition, Annan said "any further militarisation will be disastrous".
Salehi offered qualified Iranian support for Annan's efforts.
"We believe the people of Syria, like other countries, have the right to enjoy all the rights enjoyed by other world nations, such as freedom of political parties, freedom of elections, a constitution that encompasses all the wishes of a nation," he said.
"At the same time, we have announced that we oppose interference in the affairs of all countries, including Syria. The government of Bashar al-Assad has promised change to meet the demands of the people... and in fact the opportunity must be given to the Syrian government."
Annan's peace plan, presented last month, calls on the Syrian government to withdraw troops from towns and end the use of heavy weaponry.
Under the plan, both the army and opposition fighters must adhere to the ceasefire.
Walid al-Muallem, the Syrian foreign minister, on Tuesday demanded guarantees from Annan that rebels would also honour any truce.
"We will not ask the terrorist groups, which are killing, kidnapping and destroying infrastructure, for guarantees. We want Annan to give us these guarantees," Muallem said during a visit to Moscow.
Syria failed to observe a Tuesday deadline to withdraw its forces from urban areas, and activists reported fresh violence on Wednesday.
The Local Co-ordination Committees said there was shelling of several opposition-held neighbourhoods in the central city of Homs.
The Britain-based Syrian Observatory for Human Rights said "tens of army vehicles" were deploying in the southern town of Maaraba amid intense shooting.
Meanwhile, Hillary Clinton, the US secretary of state, was expected to meet Sergei Lavrov, Russia's foreign minister, in Washington.
"We will have another go at trying to persuade the Russians that the situation is deteriorating and the likelihood of regional conflict and civil war is increasing," she said on Tuesday.
Assad's forces have killed more than 9,000 people in the past year, according to a UN estimate. Damascus says rebels have killed more than 2,500 soldiers and security personnel.