France has called an EU summit with Russia on the Ukraine conflict following a prisoner exchange.
But the Dutch are unhappy that Ukraine freed a likely witness of the MH17 atrocity.
And Western countries fear that Russia plans to force undue concessions from Ukraine in the peace process despite their optimistic statements this weekend.
"The two presidents confirmed that the current momentum will allow for the holding, in the next few weeks, of a summit under the Normandy format in Paris," French president Emmanuel Macron's office said after he spoke with Russian president Vladimir Putin by phone on Sunday (8 September).
"There is an opportunity, a door opened to start making progress towards settling this conflict," French foreign minister Jean-Yves le Drian told French radio the same day.
The "Normandy format" involves France and Germany, representing the EU, as well as Russia and Ukraine.
Prior summits got Russia to agree to ceasefires and to pull out forces from east Ukraine in return for greater autonomy for Russia-occupied regions.
But the promises came to nothing and the conflict went on, claiming 13,000 lives so far, and prompting an EU and US sanctions regime.
Macron spoke to Putin one day after Russia freed 35 prisoners each, including the high-profile Ukrainian film-maker Oleg Sentsov and 24 Ukrainian sailors by Moscow and a Russian irregular fighter Vladimir Tsemakh by Kiev.
Most Western countries and institutions welcomed the development in line with France.
It was a "sign of hope", German chancellor Angela Merkel, said. "Very good news, perhaps a first giant step to peace," US president Donald Trump.
It "fills me with relief", EU Council chief Donald Tusk said.
"The European Union expects all parties to build on this momentum," the EU foreign service added, echoing Macron. "This is a step in the right direction," Nato added.
But Ukraine's release of Tsemakh, the Russian fighter, upset the Netherlands, which sees him as a "person of interest" in its enquiry into who shot down flight MH17, killing 298 people, including almost 200 Dutch nationals, five years ago.
"The government greatly regrets Mr Tsemakh's inclusion, under pressure from the Russian Federation, in this prisoner exchange," Dutch foreign minister Stef Blok said.
The exchange had "regrettably [come] at a price of the key witness in the MH17 downing case", Lithuanian foreign minister Linas Linkevicius also said.
Ukraine and the Netherlands had already questioned him, Ukraine's president, Volodymyr Zelensky, said.
But Ukraine's spy chief, Ivan Bakanov, who brokered the exchange, said Russia's demand to get back Tsemakh was a sign of guilt.
It was "fresh proof of Russian implication in the crash of MH17," Bakanov told the Interfax-Ukraine news agency.
Zelensky also called the exchange a "first step" toward ending the conflict.
But others in the Western security establishment were less sanguine than their leaders on the prospects of a Russian detente.
Critics of the deal noted that Russia continued to hold dozens of other prisoners, including 60 Crimea Tatars in Russia-occupied Crimea.
Kurt Volker, the US special envoy on Ukraine, called for "further prisoner exchanges, renewed ceasefire, and progress toward full Minsk implementation", referring to the so-called 'Minsk Accords' on ending the war.
"It would be great if we could get Russia to behave like a more normal country," US defence secretary Mark Esper also said on Saturday.
"But you also can't ignore the last many years of history where Russia has invaded Georgia, it has annexed Crimea, it is occupying parts of Ukraine, it is threatening the Baltic states," he said on a sceptical note on broader Russia relations.
The international monitors in east Ukraine, from the Organisation for Security and Co-operation in Europe (OSCE), also recorded dozens of explosions and episodes of small arms fire on the contact line in the two days prior to Saturday's exchange, indicating that little had changed on the ground.