Syrian president Bashar Assad has agreed to a new UN-brokered peace plan focusing on containing the most violent areas of the country, then expanding to the entire nation, international envoy Kofi Annan said.

At a news conference in Iran, Mr Annan said the plan must still be presented to the opposition.

But he said Mr Assad suggested trying to calm specific areas a day earlier during talks in Damascus aimed at ending the violence, which activists say has killed more than 17,000 people since March 2011.

"(Mr Assad) made a suggestion of building an approach from the ground up in some of the districts where we have extreme violence - to try and contain the violence in those districts and, step by step, build up and end the violence across the country," Mr Annan told reporters in Iran, his first step on a tour of Syria's staunchest allies.

Mr Annan later visited Iraq and met prime minister Nouri al-Maliki to discuss ways to end the 16 months of bloodshed.

The conflict in Syria has defied every international attempt to bring peace, including an earlier effort by Mr Annan, and there was no sign that the plan the UN-Arab envoy described today will be a breakthrough.

Although the government's crackdown has made Mr Assad an international pariah, he still has the support of strong allies such as Russia, Iran and China.

The international community has little appetite for military intervention of the type that helped bring down Libya's Muammar Gaddafi last year, and several rounds of sanctions and other attempts to isolate Mr Assad have done little to stop the bloodshed.

Mr Annan's latest efforts to reach out to Syrian allies suggest he sees Mr Assad's allies as integral to solving the crisis.

Mr Annan's outreach to Iran in particular appeared to oppose the approach of Washington, which has rejected Iran's participation in helping solve the crisis.

Tehran has provided Mr Assad with military and political backing for years, and has kept up its strong support for the regime since the Syrian uprising began.

US state department spokesman Patrick Ventrell said Washington doubts Iran will be able to play a constructive role.

"If the Iranian regime wants to stop giving direct material support to the Syrian killing machine ... we would welcome that. We're not at that point yet," he said yesterday in Washington.

Critics said Mr Annan's tour of Syria allies smacked of betrayal.

Rajeh Khoury, a columnist in Lebanon's leading An-Nahar newspaper, wrote that Mr Annan's actions "give the Syrian regime more time to accomplish the impossible task of crushing the uprising militarily".

"His insistence on making Iran part of the solution at a time the opposition sees Iran as part of the problem is extremely dangerous," Mr Khoury said.

Mr Annan said Tehran has offered its support to end the conflict and must be "part of the solution".

"My presence here proves that I believe Iran can play a positive role and should therefore be a part of the solution in the Syrian crisis," Mr Annan told reporters in Tehran after meeting Iranian foreign minister Ali Akbar Salehi.

He said he has "received encouragement and co-operation" from the Iranian government but did not elaborate.

Since Mr Assad took power following the death of his father, Hafez, in 2000, he has deepened cultural, political and economic ties with Iran, making it Syria's strongest regional ally. Tehran, in turn, has boosted Mr Assad's military, providing it with advanced communications technology and weapons, as well as sending elite military advisers.

All of this makes Iran unlikely to support change in Syria.

Mr Annan did not say what kind of involvement he saw for Iran in resolving the crisis, nor did Mr Salehi spell out what Tehran was willing to do to help tame the violence.

Mr Annan brokered a six-point peace plan earlier this year, but it has failed to gain traction on the ground.

Government forces and rebels have widely disregarded a ceasefire that was to begin in April, and spreading violence has kept nearly 300 UN observers monitoring the truce stuck in their hotels in Syria.

The UN envoy stressed the urgency of finding a solution to the crisis.

"If we don't make a real effort to resolve this issue peacefully and it were to get out of hand and spread in the region, it can lead to consequences that none of us could imagine," he said.

Mr Salehi said Tehran backs the rights of the Syrian people but opposes military intervention. He also blamed the conflict's increasingly chaotic violence on the meddling of foreign powers.

"Unfortunately, the unwise interference of others has caused the situation in Syria to remain critical," he said. "The worsening of the situation should not happen. It would not benefit anyone in the region."

The Syrian conflict has spilled outside the border several times. On Tuesday, the Lebanese army said shells were fired into Lebanon from Syria during an overnight exchange of fire along the countries' frontier.

The Red Cross and Lebanon's NNA state news agency said a Lebanese and two Syrians died - one from a heart attack and two others when their motorcycle hit a car in the Wadi Khaled area, where the clashes took place.

The Lebanese government decided at a Cabinet meeting last night to boost the army's presence along the volatile border, where shells fired from Syria have killed and wounded several Lebanese in the past few weeks.

Syria says the frontier is being used for smuggling weapons to rebels.