The United Nations has yet to issue invitations for the Syria peace talks scheduled for next week because key countries have not agreed on who should represent the opposition, a UN spokesman said Monday.

Meanwhile, UN Syria envoy Staffan de Mistura briefed the UN Security Council behind closed doors on his preparations for the peace talks set to be convened January 25 in Geneva.

"At this stage, the UN will proceed with issuing invitations when the countries spearheading the International Syria Support Group process come to an understanding on who among the opposition should be invited," said Farhan Haq, spokesman for UN Secretary General Ban Ki-moon.

De Mistura briefed the Security Council on his efforts to convene the talks, which were called for by a group of some 20 countries in a comprehensive roadmap to solve the conflict.

Only a week before the planned start of the talks, the 15-member Security Council was unified in its "very strong support" for de Mistura, said Elbio Rosselli, Urugauy's ambassador to the UN and current president of the council.

"No different date was considered," Rosselli said.

Vitaly Churkin, Russian ambassador to the United Nations, said that "everything is possible" regarding the start date, but "since January 25 was announced, it's important to try to stick to that date."

"We all agree that the talks must start between the Syrian government and the opposition," he said.

De Mistura told the Security Council that he has received assurances by both the Saudi Arabian and the Iranian foreign ministers that the recent diplomatic turmoil between the two countries would not affect the Syria peace process.

In Brussels, after a meeting of European Union foreign ministers, EU foreign policy chief Federica Mogherini said: "We're dealing with a war. You do not expect parties of a war to agree on something at the beginning of the process."

She said it was "the common goal of all Europeans ... to protect the fragile but very important process that we all started just a few months ago" at talks in Vienna on the Syrian conflict.

"That is the track that needs to be followed. The political talks in Geneva among the Syrian parties have to start," Mogherini said.

"I do not expect this process to be easy, or even be fast, but I expect this process to start in good faith and with enough political will inside Syria and outside Syria to allow it to start, because I really believe the Syrians cannot sustain in any possible way another, sixth year of war."

On the battlefield in Syria, regime troops fought fierce battles Monday against Islamic State militants as they mounted a push toward one of the jihadist organization's main strongholds in northern Syria, activists and a monitoring group said.

After several days of fighting south of the town of al-Bab, the clashes intensified Monday, accompanied by Russian airstrikes, activist Omar al-Halabi said from nearby rebel-held eastern Aleppo city.

The Syrian Observatory for Human Rights said the jihadists were bringing reinforcements to the fight for al-Bab, one of the two main towns in Islamic State-held eastern Aleppo province.

The area is the extremist group's last stretch of territory adjoining the Turkish border, which has in the past been one of its key supply routes.

Kurdish forces backed by US airstrikes have captured the rest of the northern border from the jihadists over the last year and recently pushed into the Aleppo region from the south-east.

Syrian state television said that troops had reached within 8 kilometres of al-Bab after advances over the weekend.

Observatory director Rami Abdel-Raham said that government forces advancing on the area from the south would need large numbers of troops to capture the town.

The United States has said that recapturing the eastern Aleppo area and the Turkish border strip from the jihadists is a priority and has been providing air support to rebels fighting Islamic State forces in the region.

However, in a sign of the complications of Syria's four-way civil war, those rebels are locked in a struggle against the Kurdish forces.

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