Four days of faltering Syrian peace talks broke up in Geneva on Friday evening with the UN’s special envoy on Syria admitting there had only been “incremental progress”, with no detailed discussions held on the main agenda items. Staffan de Mistura said he hoped talks would recommence in June, but set no specific date.

The atmosphere at the already difficult talks were soured when the Syrian government negotiator Bashar al-Ja’afari describied a US military strike in Syria on Thursday as “government terrorism” that had caused a massacre.

Russia called the strike an unacceptable breach of Syrian sovereignty and the Syrian delegation raised the matter with De Mistura in Geneva, but he refused to comment at a press conference, saying it would serve no purpose in his efforts to keep the peace talks going.

US officials said its military carried out the airstrike on Thursday against militia supported by the Syrian government that posed a threat to US forces and US-backed Syrian fighters in the country’s south.

Ja’afari said: “We discussed the massacre that the US aggressor committed yesterday in our country. This subject was widely discussed.

“The important thing is that our political ambition is higher because we want to focus on fighting terrorism represented by armed groups and the state and government terrorism happening against our country. This includes the American aggression, French aggression and British aggression, whether on civilian or military targets.”

The US strike was the second deliberate military attack on forces loyal to the Syrian president, Bashar al-Assad, since Donald Trump became US president. In April, Trump ordered cruise missile strikes in retaliation for a chemical weapons attack that Washington blamed on Damascus.

Russia, which has acted as defender of Assad and has its own troops stationed in Syria, warned: “Any military action leading to an escalation of the situation in Syria has an impact on the political process.”

Trying to focus on his efforts to secure a political transition, De Mistura said he had offered a novel proposal to the two delegations in which a group of experts, chaired by the UN, would establish a new process to write a new constitution for Syria.

He said he was pleased that the Syrian opposition delegation met with the UN experts “as an initial part of a process of expert meetings on legal and constitutional issues of relevance to the intra-Syrian talks”.

He stressed the UN was not itself seeking to write a constitution and it was for the Syrian people to do so.

The aim of the expert group would be to establish a constitution outside the context of the main peace talks, which are being held between the Syrian government delegation and the high negotiation committee (HNC), the umbrella group of the Syrian opposition, but are largely frozen.

The two sides were due to discuss four items in the week-long talks including a transitional body, detainees and a constitution. Those discussions have become bogged down with the HNC insisting there had to be an agreement that Assad would not be allowed to stay on as Syrian leader at the end of a transition.

The bulk of the Geneva talks this week instead focused on De Mistura’s idea of an expert group drawing up a constitution. The delegations fear the proposal in effect supplants their work.

Defending himself from the charge that the talks had become hopelessly bogged down, he said: “History is not written by artificial timetables, a target, a dream or a wish. We are in the most intractable conflict of recent history, but we are not going to be passive. We are not just waiting for the golden day.”