Croatia and Serbia traded ugly barbs Thursday and slammed tit-for-tat border restrictions on each other as Hungary started building yet another razor-wire barrier to keep refugees out, this time with neighboring Slovenia.

The face-off between Zagreb and Belgrade, the worst since they fought a war in the 1990s, came as thousands of migrants continued to stream through the Balkans towards northern Europe, particularly to Germany where Chancellor Angela Merkel warned the EU was a long way from tackling the worst migration crisis it has ever faced.

Crowded aboard buses and trains, thousands more migrants flooded into Austria on Sunday from countries unable or unwilling to cope with a desperate human tide fleeing war and poverty for a better life in western Europe.

Austrian police said more than 11,000 entered the country on Saturday, and another 4,700 arrived Sunday through the Nickelsdorf border post from Hungary.

Thousands of migrants sought their way through a chaotic maze of rumor and proliferating border controls in the western Balkans on Saturday.

In the latest chapter in the EU's escalating refugee crisis, Croatia, Hungary and Slovenia tussled over how to cope with a wave of refugees desperate to reach northern Europe.

Croatia on Thursday closed seven of the country's eight border crossings with Serbia "until further notice" following a massive influx of migrants and refugees, the interior ministry said.

"Traffic is banned on the border crossings of Tovarnik, Ilok, Ilok 2, Principovac, Principovac 2, Batina and Erdut," the ministry said in a statement.

Bulgaria began on Thursday deploying up to 1,000 troops to buttress its southeastern border with Turkey as hundreds of migrants remained stranded for a third day on the Turkish side, a senior official said.

"We have triggered a plan for the start of the step-by-step deployment of up to 1,000 troops along the whole Bulgarian-Turkish border (within the next week)," interior ministry chief of staff Georgy Kostov said.

Several dozen migrants entered Croatia from Serbia early Wednesday, the first to enter the EU country after Hungary sealed its borders to thousand of people entering every day, an AFP correspondent saw.

The group of mostly Syrian or Afghan men, women and children crossed the border -- which was marked by nothing more than a stone -- early Wednesday morning and were picked up by police as they walked across a field.

Serbia and Kosovo voiced satisfaction Wednesday with an EU-brokered deal on several key areas that represents a major step forward in normalizing ties between the former foes.

"We have gained more than we hoped for," Serbian Prime Minister Aleksandar Vucic told national broadcaster RTS of Tuesday's agreement.

The 100 or so refugees set off early evening, hoping that the short walk across Serbia's border into Hungary will be the last leg of their treacherous journey into the EU.

The group are mostly Syrians. Young and old, fathers carrying children on shoulders, mothers with toddlers. One man has a head wound, mugged he says by "Serbian mafia" the previous day.

Hungarian soldiers on Monday began erecting a controversial fence on the border with Serbia which is aimed at keeping out migrants.

"Works have begun on the construction of a section of the temporary border control fence," said a joint statement from the ministries of interior and defense.

The Bosnian government called Monday for a probe to track down those behind an attack on Serbia's premier during commemorations marking the 20th anniversary of the Srebrenica massacre.

Prime Minister Aleksander Vucic was chased from the Srebrenica memorial site on Saturday by a mob hurling bottles and stones in an assault that laid bare the wounds that still run deep after the Balkans conflicts of the 1990s.