Much still depends on Serbia normalising relations with Kosovo.
Serbia has moved a step closer to joining the European Union after opening talks on a series of environmental policies.
On Tuesday, Belgrade was permitted to begin discussions on climate change, energy, transport and trans-European infrastructure networks.
Serbian Prime Minister Ana Brnabić told reporters in Brussels that the move marks “a very important milestone for our European integration process”.
But the EU has warned progress on its path to EU membership still depends on Serbia normalising relations with Kosovo.
Countries wanting to join the 27-member bloc must bring their laws and regulations in line with EU standards.
Hopefuls must achieve negotiations in 35 policy areas, from the free movement of workers to taxation, or agricultural and economic policy.
Serbia has now opened 22 negotiating policy areas since its membership talks began in 2014. Tuesday was also the first time Serbia has opened four "chapters" at once.
“Serbia is taking another very important step forward in joining the European Union," said EU Enlargement Commissioner Olivér Várhelyi.
But he added that relations with Kosovo -- Serbia's former territory -- and the rule of law "remain essential to the overall pace of accession talks".
"[It is] “crucial that Serbia continues to deliver results on the ground,” Várhelyi said.
Serbia, supported by its allies Russia and China, has refused to recognise Kosovo's declaration of independence in 2008 -- as well as five current EU countries.