Hungarian Prime Minister Viktor Orban has said leaders of the Visegrad Four countries of Hungary, Poland, Slovakia and the Czech Republic will skip this weekend's mini-summit on migration in Brussels.
He said such meetings of state leaders should be organised by the European Council, the bloc's top decision-making body, not the EU Commission.
The Commission will organise the smaller summit ahead of a full EU summit next week, as an attempt to persuade other EU leaders to further curb immigration and restrict movement of asylum seekers within the European Union and help keep Merkel's ruling coalition from crumbling at home.
Speaking after a meeting in Budapest on Thursday, Polish Prime Minister Mateusz Morawiecki described Sunday's meeting as "unacceptable", adding: "We are not going to attend, they want to re-heat a proposal that we've already rejected."
His Hungarian counterpart Viktor Orban said that the meeting was "against the normal customs of the EU" and that the appropriate forum was the EU leaders' summit scheduled for next week.
The four were joined at the summit by Austrian Chancellor Sebastian Kurz, who has added his voice to calls for a tougher migration policy.
"We have to have a Europe capable of defending us," Kurz said after the meeting.
"We have to strengthen (border agency) Frontex, to protect our external border and ensure internal freedom."
The Visegrad states have traditionally taken a hardline stance on migration, rejecting any suggestion of mandatory refugee resettlement among EU members.
Brussels meeting
All 28 EU member states are unlikely to fulfil at their June 28-29 meeting the demand of Merkel's coalition partner for a bloc-wide plan to spread the burden of hosting asylum seekers more widely among the members.
Merkel hence pushed for an emergency meeting this Sunday with leaders from nine other countries - including the major arrival states Italy and Greece, as well as France, Austria, Bulgaria, the Netherlands and others.
According to agencies, Merkel is hoping a commitment by the states that do attend to do more to limit travel by asylum seekers within the bloc will help placate her coalition partners, the Christian Social Union (CSU).
Immigration figures in Europe are now much lower, with only 41,000 sea arrivals so far this year, according to UN data.
A sea route to Greece used by hundreds of thousands in 2015 was shut in 2016 by an agreement between the EU and Turkey, and use of the other main route across the Mediterranean from Libya to Italy has also tapered off as Libyan factions cracked down on people-smuggling.
But Merkel is still under pressure to obtain more commitments from other states.
Her coalition partner, the CSU from the southern Gerrman state of Bavaria, has said it would bar from Germany all asylum-seekers registered in other EU states, unless a summit of all EU leaders on June 28-29 agrees to a bloc-wide deal to share them out evenly.
Merkel's standoff with the CSU party leader, Germany's Interior Minister Horst Seehofer, has threatened the stability of her government. It has also brought back the issue of migration to the top of the EU's political agenda.
