Syrian government forces backed by Russian airstrikes Sunday pushed further into key rebel territory, ahead of peace talks planned to take place this week.

Government forces took the north-western town of Rabia, a key rebel stronghold in the mountains overlooking the regime's coastal heartland, the opposition Syrian Observatory for Human Rights said.

State news agency SANA said that troops and pro-government militiamen had captured more than 20 other "villages and points of control" in the Jabal al-Turkoman mountain range.

Sunday's gains come 11 days after government forces captured the town of Salma, a rebel stronghold in the neighbouring Jabal al-Akrad mountains.

President Bashar al-Assad's forces have recently made major advances in the north-western mountains, which lie between the regime heartland around the coastal cities of Latakia and Tartus and the rebel-held province of Idlib further inland.

The area has seen fierce fighting since October, when regime forces backed by Russian airstrikes started a widescale offensive across a 140-kilometre frontline stretching from the mountains to Syria's central desert.

Fighting elsewhere has been less conclusive, with government forces gaining ground against both rebels and Islamic State around the key northern city of Aleppo but falling back further south.

The developments on the ground came amid behind-the-scenes manoeuvring over peace talks between the government and rebels which were originally scheduled to start in Geneva on Monday.

Disagreements over who should represent the opposition have delayed the talks, although US Secretary of State John Kerry, on a visit to Saudi Arabia on Saturday, said he was confident they would get started "with a good initiative in the next day or so."

Al-Assad ally Russia is reportedly pushing for the inclusion of Syria's powerful Kurdish forces in the talks, despite resistance from the mainstream opposition and Saudi Arabia.

A source in the Kurdish-led Democratic Syria Assembly said Sunday that the secularist coalition had been promised a place in the talks.

It would participate as part of a third delegation that would also include former deputy prime minister Jamil Qadri, who is considered to be close to Moscow, the source, who asked not to be named, said.

The mainstream opposition, which last week formed a negotiating team including members of key political and military groups, has previously said it will not accept the participation of any "third party" in the talks.

Kerry met the head of the opposition negotiations committee, former Syrian prime minister Riad Hijab, during his visit to Riyadh, but gave no details about whether they had discussed the formation of the negotiating teams.

A member of the committee, who declined to be quoted by name, said Kerry had pressured it to agree to UN envoy Staffan de Mistura choosing the names of the opposition delegation.

The opposition also says that the talks cannot start without implementation of a UN Security Council resolution calling for aid deliveries to all areas in Syria and an end to indiscriminate airstrikes and shelling that cause civilian casualties.

The Observatory on Sunday said that some 164 civilians had been killed in three days of raids by Russian and Syrian planes on areas of eastern Syria controlled by the Islamic State extremist organization.

Syria's multi-sided civil war has killed some 250,000 people since 2011 and driven more than 11 million from their homes, according to estimates published by the United Nations.

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