The opposition’s High Negotiation Committee, which groups political and armed opponents of President Bashar al-Assad, has said it will not attend negotiations until the government halts bombardments, lifts blockades, and releases detainees – steps mentioned in a United Nations Security Council resolution passed last month.

Negotiator Mohamad Alloush said Kerry, who met HNC officials on Saturday, had “come to pressure us to forgo our humanitarian rights … and to go to negotiate for them”.

“There will be a big response to these pressures,” he told Reuters, without giving further details. Asked if the peace talks would go ahead this week, he said “we leave this to the coming hours”.

The government has meanwhile said it is ready 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.

(Reporting by Tom Perry; Editing by Mark Trevelyan; Editing by Mark Trevelyan)