Key European Union nations France and Germany are opposed to new EU proposals to help Italy and Greece cope with a massive influx of migrants crossing the Mediterranean.
The EU's executive Commission has a plan for sharing Syrian and Eritrean refugees over the next two years, as thousands of migrants attempt the perilous sea crossing. The system would see France and Germany host many of them.
The French and German interior ministers said in a statement that the formula "should take more properly into account the efforts already made by member states regarding international protection, and other forms of assistance".
The Commission will propose a permanent system late this year for use during high migration flows, but the ministers insist that it should "remain temporary and exceptional".
They also said that front-line countries like Italy and Greece can only expect solidarity from their partners if they "take all the legal and financial measures needed" to beef up the EU's borders to the outside world.
Italy and Greece have been accused by some EU countries of failing to properly screen newly arrived migrants, allowing them to move further north into Europe.
The interior ministers' statement came ahead of Monday's start of a two-day meeting with their counterparts from Italy, Poland, Spain and Britain, as well as senior US and EU officials, in eastern Germany.
The Commission's refugee proposals must be endorsed by member nations and the European Parliament.
Meanwhile, the EU's Frontex border agency said on Sunday that more than 5,000 migrants had been plucked from the central Mediterranean in three days aboard 25 boats coming from Libya. Seventeen bodies were recovered.
"This is the biggest wave of migrants we have seen in 2015," Frontex executive director Fabrice Leggeri said.
More than 36,000 migrants have arrived in Italy since January. The International Organisation for Migration estimates that 1,820 people have died or gone missing crossing the Mediterranean this year.