The number of migrants at the EU's borders reached a record high of 107,500 in July, officials say, as a sharp surge in expected asylum requests was reported in Germany.
Germany has seen a wave of migration from Syria and the Balkans, and now says it could receive as many as 750,000 asylum seekers this year.
The EU has been struggling to cope with migrant arrivals in recent months.
France and the UK say they will sign a deal to tackle the crisis in Calais.
Over the summer, thousands of migrants have sought to get to the UK through the Channel Tunnel from makeshift camps around the northern French city.
France's Interior Minister Bernard Cazeneuve and his British counterpart, Theresa May, say they will sign a deal there on Thursday to strengthen their countries' co-operation on security, the fight against criminal smugglers, human traffickers, and clandestine immigration.
In early August, the UK pledged to add €10m (£7m) to a fund established in September 2014 to secure the port of Calais, and initially endowed with €15m over three years.
'Third consecutive record'
EU border agency Frontex said the number of migrants surpassed the 100,000 mark in a single month for the first time since it had begun keeping records in 2008.
The Warsaw-based agency said in a statement that the figure of 107,500 migrants for July was the "third consecutive monthly record, jumping well past the previous high of more than 70,000 reached in June".
The German government had earlier forecast that 450,000 asylum seekers could arrive in 2015, but is now set to increase that to 650,000 or higher.
UN High Commissioner for Refugees Antonio Guterres said more countries in Europe should share the burden.
"It is unsustainable in the long run that only two EU countries, Germany and Sweden, take in the majority of refugees," he told German daily Die Welt.
Hungary's southern border marks the edge of the EU's Schengen zone of passport-free travel and is thus a target for migrants seeking to enter the EU.
Its government has said it will send thousands of police officers to its southern border with Serbia in its latest step to stem the flow of migrants.
Helping the refugees - by Jenny Hill, BBC Berlin correspondent
Squatting on a kerb, Barakat smiles as a volunteer hands his young son and daughter some second-hand clothes.
The family are Yazidis and fled Iraq. "They were killing all the women - what were we supposed to do?"
Nearby, hundreds of people queue outside a grey concrete building. Every so often an official comes out and shouts instructions through a megaphone.
This is Berlin's reception centre, where refugees come to register for housing and benefits. But it is volunteers who provide food, water, clothing and medical help. A doctor holds a makeshift surgery in a tent.
"It shouldn't be volunteers doing this," he says. "But the authorities never expected this many people to arrive so suddenly."
As I leave, I pass an old lady slowly pushing a trolley towards the centre. "I've come to donate clothes," she tells me. "Why wouldn't you help these people?"
More than 240,000 migrants have crossed the Mediterranean already this year, arriving on the shores of Greece and Italy before travelling on to other destinations.
In the past week alone, 21,000 migrants have arrived in Greece, according to the UN.
Germany's latest projected figures were due to be confirmed by Interior Minister Thomas de Maiziere on Wednesday and Handelsblatt reported that the number of refugees seeking accommodation around the country was surging.
Last month alone, 5,700 people applied for asylum in the northern city of Hamburg and 7,065 in the south-western state of Baden-Wuerttemberg.
"I would have to build a block of flats every day to accommodate them", Stuttgart Integration Minister Bilkay Oeney told Handelsblatt.
The numbers are far higher than the record 438,000 asylum applications in 1992 during the Bosnian crisis.
German towns have been housing refugees in tent cities and converted gyms, but as tensions increase, there has been a rise in attacks on asylum seekers.
In the east, thousands of people have marched in towns and cities in protest at asylum seekers being housed in their areas and against what they call the "Islamisation of the West".
Last month, EU member states agreed to take in 32,000 asylum seekers arriving in Italy and Greece over the next two years - fewer than the 40,000 target.
The UK is exempt from the agreement.