The mini summit, called by European Commission President Jean-Claude Juncker, groups the heads of ten EU nations, including German Chancellor Angela Merkel, plus the leaders of Albania, Serbia and Macedonia.
Bulgaria, Romania and Serbia on Saturday warned they would not accept being turned into a "buffer zone" for the tens of thousands of arrivals streaming into Europe.
"If Germany and Austria and other countries close their borders ... we will be ready to also close our borders at that very same moment," Bulgarian Prime Minister Boyko Borisov said.
Over past months, non-EU member Serbia has been swamped by migrants on their way from Greece and Macedonia to northern Europe.
Serbian Prime Minister Aleksandar Vucic, who is due in Brussels, said: "We will have difficult talks today, not pleasant for anybody, but I hope for a comprehensive solution."
EU member Hungary has already built a razor-wire fence on its border with Serbia, which forced the flood of migrants to seek a route through Croatia, causing a massive build-up.
Budapest then promptly began putting up a fence along its border with Croatia, with the migrants re-routing through tiny Slovenia.
In an interview with the Kronen Zeitung daily, Austrian Chancellor Werner Faymann said the summit would "either consolidate the unity of Europe or watch the slow decomposition of the EU."
Austrian Interior Minister Johanna Mikl-Leitner told the same daily that migrants were showing growing discontent and the police had to be "ready to react" against possible violence.
In an interview published on Sunday by the German newspaper Bild, Juncker urged countries to stop handing on migrants to neighbouring states in chaotic conditions.
Member states "must take care to uphold orderly procedures and conditions," he said.
"The European Commission expects everyone to obey the rules of the game if we don't want to put Schengen at risk," Juncker said, referring to the EU's border-free zone.
The EU is facing record arrivals with more than 60,000 people in the last week entering Slovenia, which has a population of just two million, and 48,000 entering Greece, which has a population of 11 million, according to official figures.
With winter looming, Amnesty International on Saturday warned of a humanitarian disaster if migrants are stranded at borders.
Slovenia is seeking help after becoming the main entry point into the Schengen zone when Hungary sealed its southern borders.
Visiting a registration camp in the border town of Brezice, Slovenian President Borut Pahor Pahor urged his peers to take extraordinary measures to deal with the crisis or Slovenia would take unilateral action "before it is too late."
Slovenia has asked Brussels for 140 million euros ($155 million), in addition to police back-up and logistical support.
It says that if no help emerges, it may have to build its own border barrier with Croatia.
An unprecedented 670,000 people mainly fleeing conflict in Syria, Iraq and Afghanistan have flooded into Europe so far this year in the biggest movement of its kind since World War II.
Most transit via Turkey, Greece and the western Balkans, seeking new lives in Germany and other northern EU states, creating a huge strain on those countries.
"The only goal of this long journey is to flee the disgusting war so we can start a safe life," Shaker Abara, 37, from the Syrian town of Homs, told AFP at the Berkasovo border area as he was waiting to cross from Serbia into Croatia in chilly temperatures early on Sunday.
Since last week's backlogs when thousands of refugees spent nights in the open air defying heavy rain and freezing temperatures, the situation has improved as the authorities of the two countries agreed to speed up the flow.
The 10 leaders from the 28-nation European Union who are invited to the mini summit are from Austria, Bulgaria, Croatia, Germany, Greece, Hungary, The Netherlands, Romania, Slovenia and Sweden.
The meeting takes place amid a clear split in the EU over how to best handle the crisis.
Some see the crisis mainly as a border security issue while others believe it is a humanitarian challenge requiring the bloc to show solidarity and redistribute refugees among them all.
The summit will concentrate on immediate operational problems of how to deal with the human tide, according to the draft statement seen by AFP.