Syria's government and the opposition have agreed to consider opening humanitarian access in the run-up to a peace conference that would bring the sides together for the first time, top diplomats for Russia and the US said on Monday.

Speaking in the middle of a two-day series of meetings in Paris, the Russian foreign minister, Sergei Lavrov, and the US secretary of state, John Kerry, also said they were pressing for a ceasefire and prisoner exchange between the warring sides.

But the Damascus government derided the meetings in Paris, saying in a statement they were "closer to illusions than reality and taken by people who are detached from reality and extremely far from any acceptable political logic". Russia has been one of Syria's closest allies.

Lavrov questioned whether the western-backed Syrian opposition, which has not yet agreed to attend peace talks and has limited influence inside Syria, was willing or able to carry out agreements in the face of intense infighting that has pitted al-Qaida-linked militants against more moderate factions. Nearly 700 rebel fighters have died in recent days.

"There are many terrorists in Syria and they are becoming more numerous," Lavrov said. "When we talk about the need for a ceasefire, to unblock as many settlements as possible to provide humanitarian access, all those factors are taken into account … We do not want a ceasefire which would be used by terrorist groups. Because that would be against the interests of everyone."

The opposition Syrian National Coalition is in disarray ahead of the Geneva talks on 22 January and the Paris meetings were intended to pressure it to attend. But Kerry said the opposition had nonetheless agreed to consider a ceasefire, prisoner exchange and to allow humanitarian access to thousands of people trapped behind battle lines.

Kerry acknowledged that "terrorists greatly complicated this equation", but warned: "If disorder is allowed to continue to grow, it is extremists who will benefit, and all the people who want peace and stability who will lose."

While Kerry and Lavrov agreed on several points, Washington and Moscow remain at an impasse on whether Iran, Syria's strongest ally, should attend the peace talks.

The Syrian National Coalition has struggled for credibility within Syria, as one rebel brigade after another has broken away and accused the exiled group of being out of touch. The coalition is nearing collapse, with members split on whether to attend talks that would bring the opposition to the negotiating table with President Bashar al-Assad's representatives for the first time.

Kerry said he would welcome Iran's participation in the upcoming talks but only if Tehran signs off on earlier diplomatic agreements that any transitional government in Syria would not include Assad or his close allies.

The UN did not invite Iran to the peace conference and the US has long maintained that Tehran cannot participate if it does not agree with its guiding principles.

Lavrov, however, said Iran should attend, adding that some participants have rejected parts of the earlier agreement. He did not specify, but Assad's government, which is sending a delegation, has said the president will not surrender power and may run again in elections due later this year.