European leaders including David Cameron have urged Russia to maintain the ceasefire in Syria so peace talks, which they hope will eventually lead to President Bashar al-Assad stepping down, can go ahead as early as next week.

The British prime minister, the French president, François Hollande, the German chancellor, Angela Merkel, and the Italian prime minister, Matteo Renzi, held a 50-minute call with the Russian president, Vladimir Putin, on Friday morning.

Cameron’s spokeswoman said he had stressed to Putin the importance of using the opportunity created by the ceasefire to press forward with a formal peace process. This could begin with talks in Geneva from the end of next week and end with “a transition away from Assad”.

“We welcome the fact that this fragile truce appears to be holding,” she said, adding that Downing Street hoped the ceasefire could last long enough for the process outlined in the Vienna peace agreement, struck late last year, to begin in earnest.

“Everybody on the call had a common interest in defeating Daesh [Islamic State] in Syria and tackling the Islamist threat, and therefore it is in all our interest to support a peace process in the country that can lead to a stable, inclusive government that has the support of all Syrians.”

The leaders also discussed the need to get humanitarian aid to besieged towns in Syria, and to improve conditions sufficiently to allow refugees who had fled the fighting to return home.

Speaking after the call, Hollande criticised Assad for scheduling elections in Syria next month, calling the move “provocative” and “unrealistic”.

Syrian and Russian solders stand near a car with images of Vladimir Putin and Bashar al-Assad, near Hama. Photograph: Pavel Golovkin/AP

Assad’s decree for parliamentary elections to be held on 13 April was issued shortly after Washington and Moscow announced a ceasefire plan for Syria a fortnight ago.

But Putin defended the elections, telling the leaders of Britain, France, Germany and Italy during the conference call that the plan to hold April polls “does not interfere with steps to build the peace process”. He also said the polls were in line with the existing constitution of Syria.

EU foreign ministers meeting in Paris on Friday also expressed doubts about the possibility of the peace talks starting next week in Geneva as planned if the ceasefire does not take greater hold.

The French foreign minister, Jean-Marc Ayrault, on Friday said two conditions must be fulfilled before the talks start – access for all Syrians to humanitarian aid, and full respect of the ceasefire,. The British foreign secretary, Philip Hammond, one of the most outspoken critics of Russia’s actions in Syria, said: “This cessation of hostilities is by no means perfect but it has reduced the level of violence, it has created an opportunity for some humanitarian access.”

Britain has been frustrated; there has been no agreement on how violations of the ceasefire can be monitored and handled, let alone any agreed penalties for repeated violations. There are also disputes about the territories and groups that are party to the ceasefire.

Frank-Walter Steinmeier, Germany’s foreign minister, insisted: “Humanitarian aid shouldn’t be an exception, it should be normal, and from that point of view we are still far away from a situation that is satisfying.”

Renzi was a late addition to the phone call, initiated at the instigation of European leaders. Italy is likely to head up a force between 3,000- and 7,000-strong in Libya designed to secure oil installations and attempt to force back Islamic State there. Italian media have suggested a peacekeeping mission could be under way in 10 days and represent the largest Italian military intervention since the second world war.

An unexploded cluster bomblet is seen after airstrikes by pro-Syrian government forces in the rebel-held al-Ghariyah town, in Deraa province. Photograph: Alaa Al-Faqir/Reuters

It is thought British, French and Italian special forces are operating in the country preparing the ground for a European peacekeeping force. The force would not enter Libya, a historical Italian sphere of influence, without the agreement of the Libyan government. Strenuous diplomatic efforts are under way to secure agreement on the formation of that government between two groups that are now arguing over the powers of the government and the distribution of ministries.

Russia has not stated if it opposes the idea of further western intervention in Libya, but strongly felt that the previous intervention in 2011, designed to protect Benghazi, took place after Putin was misled about the purposes of a UN security council resolution aimed at protecting innocent lives in Benghazi, but not to be used as a cover to oust Muammar Gaddafi. European diplomats acknowledge Russian antipathy to further interventions in part stem from this Libyan episode.

Inside Syria, more ceasefire breaches were reported. Three airstrikes were alleged on the edges of the city of Douma and the town of Shafuniya just after midday on Friday. Civilian casualties were confirmed but the number is not yet known. Artillery shells were also heard on the edges of the city.

Many of the disputed breaches of the ceasefire turn on whether Russia was within its rights to bomb a position on the basis that al-Nusra Front forces were present. Al-Nusra is not party to the ceasefire since it is deemed to be linked to al-Qaida.