Migrant crisis: New routes sought amid impasse in Balkans
Migrants stranded in Croatia have been making renewed efforts to head north despite moves by Slovenia and Hungary to hold them back.
Slovenian police used pepper spray on Friday night to disperse a group trying to cross from Croatia.
Hungary, meanwhile, accused Croatia of violating international law after asylum seekers were sent over the border without first being registered.
The EU, which is divided on the crisis, is to hold emergency talks next week.
Overnight, thousands of migrants trying to pass through the Balkans to reach northern EU states spent the night at railway stations or sleeping alongside roads.
Meanwhile, hundreds of people slept on a motorway near Edirne in north-western Turkey, after Turkish police stopped them from crossing the border into Greece on Friday evening.
As controls have got tighter, many migrants have strayed from transport routes, walking through cornfields to reach borders.
In a day of chaos and confusion on Friday, people were shunted from one border to another as governments remained split over how to handle the crisis.
The march by Europe's would-be immigrants is becoming a race against the changing political mood of its many nation states, says the BBC's Lucy Williamson in Croatia.
Each day now there are new routes and new rules marking the map of Europe - and without a pan-European solution, it is every nation for itself.
Thousands entered Croatia from Serbia earlier this week after Hungary fenced off its Serbian border and sealed shut the previous route north.
Hungary has taken a tough line on the migrant influx, and has now called up some army reservists to assist, according to state media, quoted by Reuters.
Croatia had initially said migrants would be welcome, but on Friday said it was overwhelmed after seeing 17,000 arrivals since Wednesday.
It now says it is regulating the flow by shutting some border crossings but will continue to give migrants passage north without making them register as refugees - and even sent a trainload of migrants across the border to Hungary, prompting Hungary to accuse it of violating international law.
"There has not been an agreement with Hungary," Prime Minister Zoran Milanovic was quoted as saying by AFP news agency.
"We have in some way compelled them to accept the refugees by sending them [to the border] and we will continue to do so."
Such an approach has angered Slovenia and Hungary, where a government spokesman accused Croatia of "intentional participation in people smuggling" and Foreign Minister Peter Szijjarto reportedly called Mr Milanovic's handling of the situation "pathetic".
For its part, Croatia has defended its conduct, with Foreign Minister Vesna Pusic telling the BBC it was behaving "humanely" and saying Hungary's decision to build a fence on its border with Croatia was "not an answer".
Asylum seekers want to reach Hungary and Slovenia, which are in the borderless Schengen Area, en route to more prosperous northern European countries - with Germany the favourite destination.
There was tension in Harmica on Croatia's northern border with Slovenia, when Slovenian riot police used pepper spray on people trying to cross the border on foot when all rail services were suspended.
But Slovenia is now putting migrants on board buses.
Bostjan Sefic, state secretary at the Slovenian interior ministry, accused Croatia of breaking the rules of both the EU and the Schengen agreement by deciding to stop registering migrants.
Europe is experiencing a huge influx of asylum seekers - many fleeing conflicts in Syria, Iraq and Afghanistan - but the EU has been strongly criticised for its failure to co-ordinate a response.
Interior ministers will hold an emergency meeting on Tuesday in another attempt to agree on relocating migrants with binding quotas for each state.
The next day, EU leaders will hold an extraordinary summit on migration.
In his invitation to leaders, European Council President Donald Tusk called the crisis a test of the EU's "humanity and responsibility".
He said migration would be a challenge for "many years to come".
Migrant crisis in Europe: Key dates
- 13 July: Hungary starts building razor-wire fence on border with Serbia
- 25 Aug: Germany says all Syrian refugees can apply for asylum there, regardless of which EU country they first entered
- 2 Sept: Image of body of three-year-old Syrian Alan Kurdi, washed ashore in Turkey, moves public opinion worldwide
- 12 Sept: Record 4,330 migrants cross into Hungary
- 13 Sept: In a switch of policy, Germany introduces border controls with Austria - other EU nations later impose their own controls
- 15 Sept: Hungary enforces tough laws on migrants crossing its border fence, prompting thousands to turn to Croatian route
- 18 Sept: Croatia transports migrants over its border into Hungary, where Hungary rushes to build new fence
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.