Serbia has compared Croatian border restrictions to racial laws enforced by a Nazi puppet state during World War Two, in a deepening row over migrants.
Croatian border police are stopping all Serbian vehicles from entering the country in protest at Serbia's decision to transport migrants to the border.
But Croatia's interior minister said the ban did not apply to Serbian people.
The Serbian authorities have responded by banning cargo traffic from Croatia.
Amid escalating tension, Serbia's foreign ministry said the measures were "discriminatory" and "can only be compared with measures taken in the past, during the fascist Independent Croatia", referring to the Nazi puppet state proclaimed in Croatia in 1941, which sided with Berlin.
Croatia's Prime Minister Zoran Milanovic, speaking in Brussels where EU leaders have been discussing a response to the migrant crisis, said Serbia's behaviour was "not normal".
Some Serbian nationals were not let into Croatia early on Thursday, but the Croatian government blamed this on a computer glitch.
Croatia has blocked cargo transport from Serbia since last week, shutting all but one of its crossings in order to prevent more migrants entering the country after a surge of almost 45,000 people arrived in a week.
The Croatian authorities are angry that Serbia has been bussing migrants straight to its border after Hungary sealed its southern border with a metal fence.
Croatia in turn sends them north across its own border with Hungary, which passes them on to Austria, but Zagreb says it cannot cope with the pace of the influx.
The border closures have damaged the economy in Serbia, which is a conduit for cargo to western and central Europe.