EU enlargement has been one of the priorities of Bulgaria's EU presidency in the first half of this year.
Croatia, the newest EU member, will be heading the 28-member bloc in the first six months of 2020.
Last week a summit of the EU and six Western Balkan countries was held in Sofia and high on the agenda of the meeting was, for the first time after 15 years, the topic of enlargement.
"That was an excellent message to neighbouring countries to continue with reforms and provided impulse to the EU to help in those processes, notably regarding infrastructure connectivity," Plenkovic told a news conference.
Borisov said that the Balkans was faced with what he described as numerous difficult situations, citing in that context
Bosnia and Herzegovina, where, he said, various countries other than EU members exerted significant influence, the issue of Kosovo and the name dispute between Macedonia and Greece, which has been blocking Macedonia's integration with the EU and NATO.
Those situations have to continue to be dealt with and Croatia will definitely contribute significantly to that process because without its support, we cannot move on, said Borisov.
Accession to the euro area and the Schengen zone of free movement of people and goods is a joint ambition of Croatia and Bulgaria, which are also partners in NATO.
Plenkovic said that by 2019 Croatia planned to complete technical preparations for joining the Schengen area and then wait for a political decision to join the area, which is particularly important for the protection of the EU's external borders against illegal migrations.
The EU-Balkan summit in Sofia was the first such meeting after 15 years, the last one having taken place in Thessaloniki in 2003. The EU confirmed the membership prospects of Western Balkan countries at the summit, but made no concrete promises of accession.
It did, however, stress its readiness to step up investments for the sake of better connectivity between the region and the EU and regional connectivity.
Borisov today also attended the unveiling of a monument to Bulgarian poet, novelist and playwright Ivan Vazov in Zagreb, at a ceremony that was also attended by Croatian Foreign Minister Marija Pejcinovic Buric.
Text: Hina