Two buses with tinted windows and a police escort left a Moscow jail on Saturday morning carrying an undisclosed number of people as a major prisoner exchange between Russia and Ukraine appeared to begin, a Reuters witness and state TV said.

Expectations of the swap, described as imminent by the two countries’ leaders in recent days, have been running high after lengthy negotiations.

The event could help build confidence between the two countries and allow them to start negotiating seriously about other things. But any road to a full rapprochement is likely to be long and complex.

Ukraine still wants the Crimea region back, which Russia annexed from it in 2014, and Russian-backed separatists continue to control a swath of eastern Ukraine in a conflict that has claimed over 13,000 lives.

Still, the exchange will be seen in some quarters as a win for Ukrainian President Volodymyr Zelenskiy who swept to power earlier this year promising to bring Ukrainian prisoners held in Russia home.

He has also pledged to end the war in the east, which has continued to see regular, low-level clashes despite a ceasefire signed in 2015.

The swap could set the stage for the two countries to begin serious negotiations about eastern Ukraine even though serious differences remain. French President Emmanuel Macron has been pushing for a summit to discuss the issue with Russia, Ukraine and Germany and France.

Russian President Vladimir Putin said on Thursday the prisoner exchange would be “a good step forward towards the normalization (of relations),” adding he expected large numbers of prisoners to be involved.

It is unclear how many captives are expected to be exchanged as part of the deal, with officials on both sides making conflicting statements in recent days.

Among the captives held in Moscow are 24 Ukrainian sailors detained by Russia during a clash near Crimea last year, as well as a filmmaker, Oleg Sentsov, whose release Kiev has been hoping to secure.

It is not clear how many Russian prisoners are held in Ukraine. Among those handed over to Moscow as part of the exchange could be Volodymyr Tsemakh, a former commander of separatist forces in Ukraine’s east, who is suspected of involvement in the downing of Malaysia Airlines flight MH17 over eastern Ukraine in 2014, killing 298 people.

He was released on bail by a Ukrainian court on Thursday.

Dutch prosecutors have urged Kiev not to allow Tsemakh to travel to Russia, fearing this could jeopardize the investigation into Flight MH17, which was en route from Amsterdam to Kuala Lumpur.