Long-awaited talks designed to end the civil war in Syria and elect a new government within 18 months are due to start on Monday, despite Syrian calls for the future of president Bashar al-Assad to be excluded from the agenda.

As United Nations special envoy Staffan de Mistura battled to keep the talks on track, the US secretary of state, John Kerry, described the Syrian government’s remarks about Assad as a spoiler, and urged Russia to haul Damascus into line.

“It’s a provocation ... a bad sign and doesn’t correspond to the spirit of the ceasefire,” the French foreign minister, Jean-Marc Ayrault, added at the end of talks with western foreign ministers in Paris on Sunday.

The Geneva proximity talks are due to start after a surprisingly successful two week cessation of hostilities, but with the main Syrian opposition – gathered under the umbrella of the high negotiations committee (HNC) – still facing military reverses on the battlefield at the hands of the al-Qaida-linked al-Nusra Front.

A balancing act: Staffan de Mistura will have to weigh up the interests of all groups to keep negotiations together in Geneva. Photograph: Denis Balibouse/Reuters

In common with Islamic State (Isis), al-Nusra are not party to the two week long truce.

De Mistura will regard the talks success if he can keep all the parties to the talks in Geneva, including their many sponsoring nations, in negotiation talks until the first planned break at the end of the month.

The pause is designed to give all sides to the conflict time to go back to their respective interest groups and seek a mandate for a further round of talks. The two sides are not expected to meet face-to-face, and De Mistura will shuttle between them, trying to avoid a breakdown over the issue of whether Assad agrees not to stand in future elections.

“We will not talk with anyone who wants to discuss the presidency ... Bashar al-Assad is a red line,” Syrian foreign minister, Walid Muallem, told a Damascus news conference on Saturday. “If they continue with this approach, there’s no reason for them to come to Geneva.” He said the purpose of the talks was to agree a new constitution and the modalities for a referendum to sanction that constitution.

The HNC has repeatedly called for Assad’s departure. Chief opposition negotiator Mohammad Alloush, already in Geneva, said: “We believe that the transitional period should start with the fall, or death, of Assad. It cannot start with the presence of the regime, or the head of this regime still in power.”

Separate working groups will also be meeting to discuss progress on the cessation of hostilities and disputes over access to humanitarian aid. One of the earliest issues is likely to be HNC demands for the release of political prisoners as a gesture of good will.

Meeting fellow foreign affairs ministers in Paris ahead of the talks in Switzerland, Kerry said it was very significant that the cessation of hostilities had reduced violence by 80-90%. That estimate is not shared by Russia or opposition forces that claim there have been hundreds of breaches. .

The UN is also struggling with the issue of the status of the Syrian Kurds at the talks. The Turkish government is strongly opposed to their presence, but European governments are more ambivalent. Russian foreign minister, Sergey Lavrov, urged partners to “bring the Kurds on board” during the second round of the negotiating process in Geneva.

The Free Syrian Army has faced attacks from the al-Nusra Front in recent days, despite the cessation of hostilities. Photograph: Hussein Malla/AP

“Starting the talks without the participation of this group … is a manifestation of weakness on the part of the international community, as only the Turks are blocking the invitation for the Kurds, specifically the Democratic Union party,” he said.

Meanwhile, the al-Nusra Front seized bases and weapons from the western-backed Free Syrian Army in overnight fighting in north-west Syria, according to the Syrian Observatory for Human Rights said on Sunday.

Director Rami Abdulrahman said the al-Nusra Front had also detained dozens of members of the rebel 13th Division – one of the factions that has received foreign military aid – capturing US-made anti-tank missiles.

The 13th Division, which is led by rebel commander Ahmed al-Seoud and fights under the banner of the Free Syrian Army, confirmed on Twitter that al-Nusra Front fighters attacked its bases and seized weapons, but gave no further details.