Hungary's army has begun exercises to prepare for a possible future role in guarding the southern border to try to stem the influx of migrants.
Budapest plans to send soldiers to help police at the border where thousands of migrants arrive from Serbia every day.
A new razor-wire barrier is already being built along the frontier. MPs are expected to vote on stricter border controls later this month.
The plans for militarisation have been criticised by human rights groups.
Meanwhile the authorities in Hungary have been told to expect 40,000 more migrants by next week.
The BBC's Bethany Bell at the Austria-Hungary border reports that hundreds more people have arrived there and that 3,500 had crossed into Austria overnight.
Many migrants are fleeing conflicts in countries like Syria and Libya and are trying to travel through Hungary to Germany, Austria and Sweden - wealthier EU nations with more liberal asylum laws.
In other developments:
- Another group of 2,500 migrants boarded a ferry hired by the Greek government early on Thursday morning to transport them from the island of Lesbos to the mainland
- Denmark earlier suspended all rail links with Germany and temporarily closed a motorway after police stopped hundreds of migrants trying to reach Sweden at the border. Trains are to resume after some migrants agreed to register in Denmark and others were driven away in cars
- The Irish government has announced it will take in 4,000 refugees - the figure includes 1,120 people Ireland has already agreed to receive
- The European Commission earlier proposed that 120,000 additional asylum seekers should be shared out between 28 members of the EU
The Hungarian army launched the Decisive Action exercises on Wednesday to prepare for its possible new role, pending a vote in parliament.
"It is our job to make sure Hungary is defended," Gen Tibor Benko said.
Last month Nils Muiznieks, the Council of Europe Commissioner for Human Rights, criticised the country's plans as "ill-advised", saying "militarisation of borders is [the] wrong answer to migration".
Hungary has recently completed a 175-km (110 mile) razor-wire fence along its border with Serbia, and is building an additional barrier.
Prime Minister Viktor Orban this week pledged to speed up the construction of the new barrier.
However, the measures have so far failed to stop thousands of people getting into the country from Serbia.
Hungary has become a key point on the journey north for the migrants, with more than 150,000 people arriving this year.
A surge of migrants fleeing conflict and hardship in Africa and the Middle East has pushed north through Europe over the past few weeks.
On Wednesday, European Commission President Jean-Claude Juncker announced plans for a "swift, determined and comprehensive" response through a quota system.
In a "state of the union" annual address, he said tackling the crisis was "a matter of humanity and human dignity".
Among Mr Juncker's proposals:
- EU member states to accept their share of an additional 120,000 refugees, building upon proposed quotas to relocate 40,000 refugees which were set out in May (though governments then only actually agreed to take 32,000)
- A permanent relocation system to "deal with crisis situations more swiftly in the future"
- Commission to propose list of "safe countries" to which migrants would generally have to return
- Efforts to strengthen the EU's common asylum system
- A review of the so-called Dublin system, which states that people must claim asylum in the state where they first enter the EU
- Better management of external borders and better legal channels for migration
Next steps for EU leaders:
14 Sept: Special meeting of EU interior ministers on refugee crisis, with Juncker proposals on agenda
15-16 Oct: EU leaders' summit, with refugee crisis high on agenda. European Parliament then to decide on any new asylum measures with EU governments
Early 2016: EU proposals for better management of legal migration to EU due
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.