The Syrian government will not make any new concessions in future peace talks, a senior official has insisted.
“There will be a big response to these pressures”, he told Reuters, without giving further details.
Syrian opposition figures say groups opposed to President Bashar Assad are still divided ahead of peace talks scheduled to begin in Geneva next week.
The UN special envoy for Syria, Staffan de Mistura, has the final say on the delegation and who will receive invitations to attend the talks. They had been due to begin in Geneva on Monday, but a Western diplomat said earlier on Sunday that they were unlikely to begin before Wednesday.
However, officials were still undecided about who would represent the Syrian opposition at the table.
In November, Alexander Aksenyonok, a former Soviet ambassador to Syria, who is still involved in political negotiations in the country, told the Guardian that it was unlikely Assad would be able to stay on.
The United States is mounting a last-ditch effort to salvage stalled political talks aimed at ending Syria’s almost five-year civil war, deploying high-level delegations to Turkey and Saudi Arabia this weekend to head off possible boycotts by the Syrian opposition and one of its major backers, Ankara.
The rebels’ statement was signed off by groups including the powerful Jaysh al-Islam, whose politburo member Mohamad Alloush is the chief negotiator on the High Negotiations Committee – a body drawn up by civilian and armed opposition groups after a meeting in Riyadh last month.
Damascus has been fighting foreign-backed militant groups in the country since 2011. Russian Federation wants the opposition delegation expanded to include groups such a Kurdish faction that controls wide areas of the northeast.
The suspected Russian airstrikes Saturday hit eastern Syria, with the Syrian Observatory for Human Rights reporting that nine children were killed in Khasham, an area controlled by the Islamic State group.
The opposition accuses the Kurds of cooperating with Damascus, a charge they deny. He also accused Jaysh al-Islam of fostering the “same mentality” as al Qaeda and Islamic State.