Arab Gulf leaders have held talks with Russia's foreign minister in Moscow as part of renewed efforts to stop the war in Syria.
Thursday's meeting between Sergey Lavrov and the Gulf Cooperation Council (GCC) foreign ministers comes as rebels in northern Syria continue to make slow progress in their campaign to evict the Islamic State of Iraq and the Levant (ISIL, or ISIS) armed group from its stronghold in the city of Raqqa.
Al Jazeera's Rory Challands, reporting from Moscow, said Syria is expected to dominate the meeting.
However, there are "still very big differences" between the two sides - which are meeting for the fourth time - on the issue of Syria, he said.
"One of those is about the fate of Syrian President Bashar al-Assad. Another has to do with the different opposition groups that are fighting in Syria at the moment.
"There are two particular ones - Ahrar al-Sham and Jaish al-Islam - which Russia wants to be listed as terrorist organisations. But these two groups have been vetted by the Saudis as part of the High Negotiations Committee, which is the key negotiating bloc of the opposition movement inside Syria.
"This is a difficult point between the GCC and Russia."
The diplomatic developments come against the backdrop of ongoing fighting in northern Syria, with a Kurdish-led coalition of armed groups making advances in their fight against ISIL.
The Syrian Democratic Forces (SDF), a US-backed coalition of Kurdish and Arab fighters headed by the Kurdish People's Protection Units (YPG), have taken control of four villages near the town of Ain Issa, about 60km from Raqqa.
"They're making slow progress," said Al Jazeera's Stefanie Dekker, reporting from Gaziantep near the Syrian-Turkish border on Thursday.
READ MORE: Kurdish-led forces advance in Raqqa offensive
"The SDF are saying they have taken around 6km of land so far since these events started two days ago, but it is proving quite a difficult fight."
She said there have been casualties on both sides.
"We spoke to the spokesperson of the SDF yesterday ... [who] emphasised that this is not a push for the city as of yet - but just trying to move forward towards the city in the northern countryside," our correspondent said.
ISIL has imposed strict rule in Raqqa - home to more than 220,000 people before the Syrian conflict - and committed atrocities against the civilian population since its takeover of the city more than two years ago.
Source: Al Jazeera