European Union ministers are meeting to try to resolve a dispute over how to relocate 120,000 asylum seekers who have recently arrived in Europe.
Some central European states have resisted calls for EU members to accept mandatory quotas.
Whatever is decided, the UN says the EU's plans will not be enough.
The migrants are part of 500,000 to have arrived by sea this year so far. Germany says it expects at least 800,000 this year.
The arrival of hundreds of thousands of migrants has created deep EU divisions.
Home affairs ministers meeting in Brussels on Tuesday are hoping to reach agreement which would be ratified by EU leaders on Wednesday.
Who are the 120,000?
- 120,000 migrants "in clear need of international protection" to be resettled from Italy, Greece, Hungary to other EU member states
- 15,600 from Italy, 50,400 from Greece, 54,000 from Hungary, it is unclear though how many are still in Hungary
- Initial screening of asylum applicants carried out in Greece, Hungary and Italy
- Syrians, Eritreans, Iraqis prioritised
- Financial penalty of 0.002% of GDP for those member countries refusing to accept relocated migrants
- Relocation to accepting countries depends on size of economy and population, average number of asylum applications
- Transfer of individual applicants within two months
Source: European Commission
Hungary, Poland, Slovakia and the Czech Republic all oppose the idea of obligatory quotas, promoted by Germany which has accepted large numbers of migrants.
German Chancellor Angela Merkel said two weeks ago that obligatory quotas were "a first step" towards a more permanent scheme to deal with the influx.
But mandatory quotas have now been dropped, diplomats say, and a voluntary relocation scheme is now on the table.
At the scene: Bethany Bell, BBC News, Nickelsdorf on Austria-Hungary border
Refugees and migrants have been walking over the border from Hungary. The young men come first, waving and asking: "Is this Austria?"
There are cheers when they are told where they are.
The families follow, a father holding the hands of his two young children, a mother carrying her baby, then a man pushing a boy in a wheelchair.
Many are from Syria - others say they are from Afghanistan, Iraq and Pakistan. Some walk to the reception centre where the Red Cross has food and clothes for them. But others go straight to the queues for the buses which will carry them away from the border.
Some apply for asylum in Austria but most say they want to go on to Germany.
Most of those arriving in the European Union are from war-torn Syria, the European Commission says.
The relocation scheme would prioritise migrants recognised as "in need of international protection" - those from Syria, Eritrea and Iraq, according to EU data.
The 120,000 would be transferred from Hungary, Greece and Italy - the states where most migrants have been entering the European Union.
The UK, under an opt-out, would not be part of the relocation scheme but has already agreed to take 20,000 migrants directly from countries bordering Syria over the next five years.
The Irish Republic and Denmark, with similar opt-outs, have agreed to take part in the EU scheme.
The UN Refugee Agency has warned that the relocation scheme will be insufficient given the large numbers arriving in Europe.
"A relocation programme alone, at this stage in the crisis, will not be enough to stabilise the situation," spokeswoman Melissa Fleming said.
The number of those needing to be relocated will probably need to be revised upwards significantly, she said.
Newcomers continued arriving in southern Europe on Tuesday in the hope of making their way north to Germany and Scandinavia.
Hundreds are crossing the Hungarian border into Austria - and about 4,000 are reaching the Greek island of Lesbos every day.
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.