U.N. peace envoy Kofi Annan waded into big power politics on Tuesday, insisting regional heavyweight Iran should be involved in efforts to find a peaceful solution to the Syria crisis despite the West's firm rejection of a role for Tehran.

The United States and its NATO and Gulf Arab allies are opposed to involving the Islamic Republic, which strongly backs Syrian President Bashar al-Assad and is regarded as their main adversary in the Middle East.

Such diplomatic rifts have prevented effective international action to end the 16-month-old conflict in Syria.

"Iran has a role to play. And my presence here explains that I believe in that," Annan said after talks in Tehran with Foreign Minister Ali Akbar Salehi.

"I have received encouragement and cooperation with the minister and the (Iranian) government," Annan said.

The former U.N. secretary general said Iran had made clear that if the crisis got 'out of hand and spread to the region, it could lead to consequences that none of us can imagine'.

Washington repeated its support for Annan's broader peace plan for Syria but gave no sign it was easing its opposition to Iran playing a role.

"I don't think anybody with a straight face could argue that Iran has had a positive impact on developments in Syria," White House spokesman Jay Carney told reporters.

Russia, which along with China opposes any external move to tip the balance against Assad, has said Iran should be involved. Moscow on Tuesday suggested hosting regular meetings of an 'action group' which would include the Syrian opposition.

Annan said after talks in Damascus on Monday that Assad had suggested easing the conflict on a step-by-step basis, starting with districts that have suffered the worst violence.

After his talks in Tehran, Annan met Iraqi Prime Minister Nouri al-Maliki in Baghdad. Assad and Maliki both have close relations with Iran, a Shi'ite Muslim power vying with Sunni Gulf Arab states for predominance in the wider region.

Annan was trying to enlist the support of regional powers in efforts to revive his moribund plan for ending the crisis, in which rebels are fighting to topple the authoritarian Assad.

Annan said the first attempt to call a truce on April 12 had failed. He underlined the risk of the conflict 'spilling over' to neighboring states and noted that the mandate of U.N. monitors in Syria expires on July 21.

Annan said Assad had proposed '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'.

He said he needed to discuss the proposal with the Syrian opposition and could not give further details. It was not clear when he planned to do so.

Opposition leaders say there can be no peaceful transition unless Assad, who crushed popular protests from the moment they began, relinquishes power first. Assad, whose family has ruled Syria for 42 years, has ruled out leaving office in such a way.

Annan is due to brief the U.N. Security Council in New York on Wednesday. The 15-member council must make a decision on what to do with the U.N. mission in Syria, known as UNSMIS, before July 20 when its mandate expires. It is due to vote on July 18.

The Council approved in April the deployment of up to 300 unarmed military observers to Syria to oversee a ceasefire, part of a six-point peace plan proposed by Annan, but it was never honored and the monitors are now confined to hotels.

COME TO MOSCOW

Annan originally wanted Iran to be part of the major power 'action group' meeting in Geneva on June 30, but it was vetoed.

Syria's weightiest ally Russia proposed what sounded like an alternative to the Western-backed, anti-Assad 'Friends of Syria' forum, with an offer to visiting Syrian opposition groups to host regular meetings of Annan's own 'Action Group' of states, which is more balanced between pro- and anti-Assad influences.

The Syrian National Council (SNC) - the main opposition umbrella group in exile - was due to hold talks on Wednesday with Russian Foreign Minister Sergei Lavrov.

French Foreign Minister Laurent Fabius did not seem enthusiastic about the Russian proposal when reporters asked him about it during a visit to Beijing.

"We will see. There must be a need for such a meeting for it to take place. After Kofi Annan's visit to Damascus, would it be more or less necessary? I can't say," Fabius said.

"The Action Group is a group for action, to make progress. So we will, if necessary, hold such a meeting. Mr. Lavrov said it could be in Moscow or Geneva. What matters is whether a meeting will be useful or not."

It was agreed at the Geneva meeting that a transitional government should be set up in Syria, but the major powers remain at odds over what part Assad might play in the process.

Russia says no transition plan can pre-suppose that Assad will step down. The West and allied Gulf Arab states say he must go, and the Syrian opposition say that is their basic condition.

DEATH TOLL MOUNTS

The activist Syrian Observatory for Human Rights said at least 17,129 people have been killed in Syria's increasingly sectarian revolt, pitting rebels from the Sunni Muslim majority against Assad's Alawites, related to Shi'ite Islam.

It said 11,897 civilians or armed insurgents had been killed by Assad's forces, but that it could not determine how many fell into each category.

It also estimated that 884 defectors had been killed. The Observatory put the death toll among Syrian security forces loyal to Assad at 4,348.

The Syrian government has not given a death toll for security forces for several months but Assad said last week that most of the victims of the uprising were government supporters.

The Observatory said several towns were shelled on Tuesday in the northerly Aleppo and Idlib provinces, which border Turkey.

In Latakia province, further west but also close to the Turkish border, Syrian forces fired on Jabal al-Akrad in an attempt to regain control from rebels infiltrating from Turkey.

In Deir al-Zor, on the eastern road to Iraq, a volunteer medic was killed and at least four soldiers died in fighting.

Clashes were also reported overnight in Deraa, along the border with Jordan, and gunfire and explosions rocked the cities of Homs and Hama and the central town of Rastan.

The SNC said it was time for the United Nations to declare a humanitarian emergency in Syria, where the U.N. says one and a half million out of a population of 22 million have been affected by the conflict.

A Syrian Arab Red Crescent aid worker, Khaled Khaffaji, died on Tuesday a day after he was shot in a clearly marked ambulance in the town of Deir al-Zor, the organisation said. He was the fifth member of the group's staff to be killed in the conflict.

"The loss of Khaled is completely unacceptable," said Syrian Arab Red Crescent President Abdul Rahman al-Attar.

"All sides must respect health-care workers and the Red Cross and Red Crescent emblems, and allow Syrian Arab Red Crescent volunteers to provide assistance unhindered and in safety."

LEBANON THREATENED

Three people were killed when Syrian mortars hit villages in neighboring north Lebanon. Locals said they were under fire for five hours overnight, after sporadic shelling in the area lasting several days.

It was the second such fatal attack in three days. Three people were killed inside Lebanon by mortar fire at the weekend.

Lebanon has long been a political battleground for bigger regional powers. Damascus had a major military presence in its smaller coastal neighbor for 29 years

Assad withdrew his troops in 2005, but Damascus is still the main external player in the delicately balanced sectarian politics of a country torn apart by a 1975-90 civil war.

The border has become more volatile in the past month, raising fears that Lebanon could be drawn into Syria's conflict. It is mirrored in Tripoli, Lebanon's second largest city, where armed Sunnis and Alawites have fought twice this year.

There is concern that a proxy conflict is being fought out, with Saudi Arabia and Qatar pouring funds into Tripoli through Salafi Islamist groups - increasingly powerful in Lebanon - against Lebanon's influential Shi'ite Hezbollah and Amal factions, which back the Alawite-led government in Damascus.