UN-Arab League envoy Lakhdar Brahimi said Tuesday it was up to Syrians to shape their political transition in hoped-for peace talks, after warning of the potential "Somalisation" of the country.

He spoke as fighting prevented chemical weapons inspectors from visiting two sites, although UN chief Ban Ki-moon said the mission to destroy Syria's arsenal by mid-2014 was still on track.

Meanwhile, the World Health Organisation confirmed an outbreak of polio in Syria -- the first since 1999 -- saying laboratory tests had confirmed the disease in 10 of 22 suspected cases.

Brahimi has been seeking to build on the momentum of last month's US-Russian deal to eradicate Syria's chemical weapons in order to launch the so-called Geneva II peace talks proposed for next month.

But the talks have been cast into doubt by the increasingly divided opposition's refusal to attend unless the departure of President Bashar al-Assad is on the table, a demand rejected by Damascus.

In brief remarks to reporters outside a Damascus hotel, Brahimi insisted the Geneva talks would be "between the Syrian parties, and it is the Syrian parties who will determine the transitional phase and what comes after, not me."

In an interview with French website Jeune Afrique published on Monday, Brahimi had said Assad could contribute to the transition to a "new" Syria but not as the country's leader.

Brahimi had angered the regime during his last visit to Syria in December when he called on Assad to hand over power to a transitional government.

More than 115,000 people are estimated to have been killed in Syria's 31-month conflict, which erupted after the regime launched a brutal crackdown on peaceful pro-democracy protests.

In the latest blow to peace efforts, 19 Islamist rebel groups said Sunday that anyone who attends the Geneva talks would be committing "treason" and could face execution.

The warning added to doubts over whether any agreement reached by Syria's external opposition could be implemented on the ground.

In recent months rebel groups have clashed among themselves, and several prominent brigades have rejected the National Coalition -- the main Western- and Arab-backed opposition group -- which is to meet on November 9 to decide whether to take part in the Geneva talks.

Brahimi, a veteran troubleshooter, acknowledged to Jeune Afrique that "the entire world will not be present" at the talks, but said the alternative to a political settlement could be a failed state in the heart of the Middle East.

"The real danger is a sort of 'Somalisation,' but even more deep and lasting than what we have seen in Somalia."

In the latest measure of Syria's disintegration, the WHO confirmed a cluster of polio cases in the northeastern province of Deir Ezzor, all of whom were children under the age of two.

Before the outbreak of the civil war around 95 percent of all Syrian children were vaccinated, but since then some 500,000 children have gone without, according to the United Nations.

The intensity of the fighting in Syria has slowed the unprecedented international mission to dispose of a vast chemical arsenal in a country torn by civil war.

The Hague-based Organisation for the Prohibition of Chemical Weapons said Monday its inspectors had been unable to reach the last two of 23 disclosed chemical weapons sites for "security reasons."

UN chief Ban Ki-moon said the inspectors were still on track to destroy Syria's chemical weapons production equipment by November 1, the first major deadline of a timetable set out by the Security Council.

Ban said Damascus has extended "consistent, constructive" support to the mission but warned that "the job is far from complete and much important work remains to be done."

On the battlefield, Kurdish fighters advanced across the northeast after seizing an Iraq border post from jihadists over the weekend, according to the Syrian Observatory for Human Rights, a Britain-based group.

It said the Kurds had seized two villages in Hasakeh province and surrounded a rebel brigade that is part of the Western-backed Free Syrian Army, forcing it to surrender a tank, rocket launchers and vehicle-mounted canons and heavy machine guns.

As the conflict has grown increasingly muddled, the Kurds have fought both the army and other rebel groups in a bid to carve out an autonomous zone modelled on the Kurdish region of Iraq.

Jihadists from the Islamic State of Iraq and the Levant have sought control over the border to facilitate the flow of fighters and arms, as it launched attacks in both Iraq and Syria.