UN-Arab League envoy Lakhdar Brahimi visited Syria Monday to seek support for peace talks, as violence prevented international inspectors from visiting two chemical weapons sites, in their first setback.
However, UN chief Ban Ki-moon said Syria was still on target to destroy its chemical weapons production equipment by November 1, a key step in dismantling its entire arsenal by the middle of next year.
The Syria leg of Brahimi's regional tour to drum up support for the so-called Geneva II talks is the most sensitive as he needs to persuade a wary regime and an increasingly divided opposition to attend.
In an interview with a French website published Monday, Brahimi said President Bashar al-Assad could contribute to the transition to a "new" Syria but not as the country's leader.
"What history teaches us is that after a crisis like this there is no going back," he told the Jeune Afrique website ahead of his first visit to Syria since December.
He also said the US-Russian accord to dismantle Syria's chemical weapons had transformed Assad from a "pariah" into a "partner" and further convinced his supporters of his ability to prevail.
Brahimi has been on a regional tour, also to include Turkey but not Saudi Arabia, which opposes the peace initiative and takes a hard line against President Bashar al-Assad's regime.
On Sunday, Brahimi said in Tehran that it was "necessary" for Iran, a key ally of the Damascus regime, to take part in the Geneva conference slated for next month and aimed at ending Syria's two-and-a-half-year conflict.
The initiative's backers, Washington and Moscow, have struggled to win the support of the warring parties in Syria, where more than 115,000 people are estimated to have been killed in the conflict.
In the latest blow to peace efforts, 19 Islamist groups fighting to topple Assad issued a statement Sunday saying anyone who attends the Geneva talks would be committing "treason" and "would have to answer for it before our courts," implying they could face execution.
Russia denounces 'outrageous' threats
Russia on Monday issued a stinging rebuke to the rebels.
"It is outrageous that some of these extremist, terrorist organisations fighting government forces in Syria are starting to make threats," Foreign Minister Sergei Lavrov said in televised comments.
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Thomas Pierret, an expert on Islam in Syria, said the rebel statement came from a wide range of opposition groups from radical Salafists to moderates who form the backbone of the mainstream rebel Free Syrian Army.
"So you can say that Geneva II is almost totally rejected within rebel ranks," said the expert at Scotland's Edinburgh University.
"If some of the opposition does take part and reaches an accord, it will be worthless."
Under pressure from its Western backers to attend, the National Coalition opposition group is to meet on November 9 to decide whether to take part.
But it has insisted it will only do so if there are guarantees Assad will step down, and its leader Ahmad Jarba has also said no talks can take place unless the regime frees women and children from its jails.
Assad has repeatedly rejected negotiations with any group tied to the rebels or to foreign states.
The intensity of the fighting in Syria has meanwhile slowed the unprecedented international mission to dispose of a vast chemical arsenal in a country torn apart by civil war.
The Hague-based Organisation for the Prohibition of Chemical Weapons said Monday its inspectors had been unable to reach two remaining chemical weapons sites due to the war.
Inspectors were supposed to have visited all sites declared by Syria by Sunday as part of their mission to oversee the elimination of the country's chemical weapons by mid-2014.
But the OPCW said that while 21 of 23 sites had been inspected, "the two remaining sites have not been visited due to security reasons."
The disarmament mission is part of a US-Russian deal agreed last month that headed off US military strikes on Syria.
On the battlefront, Syria's army regained control of the Christian town of Sadad in the central province of Homs after days of fighting against rebels and jihadists, Syria's state news agency SANA reported Monday.
The Syrian Observatory for Human Rights confirmed the report, adding that anti-government forces had withdrawn to Mahin, the scene of fierce fighting over the past week for control of a large arms depot.
In the eastern province of Deir Ezzor, meanwhile, a Sunni Arab MP was kidnapped by Islamist fighters during clashes with members of his pro-regime tribe, the Observatory said.