The leaders of Britain, France, Germany and Turkey on Tuesday discussed the European migration crisis and the situation in Syria.
The call between the four nations came after a Turkish decision last month to reopen its border for refugees trying to reach Europe.
That led to about 13,000 migrants becoming stuck on the border with Greece and Bulgaria in Turkey’s Edirne province, the UN estimates. Many of the refugees are Syrian.
Greece has taken a hard line on the migrants, using tear gas to stop them crossing the border. Athens also suspended asylum applications for a month.
“The leaders welcomed the recent ceasefire in Idlib,” a Downing Street spokesman said.
“They also condemned the regime and its backers for causing the ongoing humanitarian crisis and the mass displacement of people from and within Syria.”
Clarifications were requested by the European countries from Turkish President Recep Tayyip Erdogan on migration, Nato and EU relations “to achieve clearer and peaceful relations”, the French presidential office said.
The leaders originally planned to meet in Istanbul but instead spoke by phone because of the rapid spread of the coronavirus.
In 2016, the EU tried to end the surge of refugees entering the bloc by promising Turkey €6 billion to help house the migrants there.
Only about €3.2bn of that sum has been paid.
Turkey hosts 4 million refugees, most of whom are Syrian.
Ankara is demanding more financial assistance from the EU but critics say Mr Erdogan is trying to politicise migration to gain more concessions from Brussels.
On Sunday, Syria’s civil war, which has displaced 11 million people and killed more than 384,000, entered its 10th year.
This month, Mr Erdogan visited Moscow to try to make progress on Syria, but left only with a limited ceasefire, which was quickly broken.
A few days later, on March 9, he met EU leaders to discuss his demands for extra financial support but no agreement was reached.
During Tuesday's call, the four leaders also discussed the coronavirus.
After the call, Mrs Merkel said people who were not EU citizens could no longer enter the bloc for at least 30 days, in a bid to contain its spread.
Germany has been badly affected by the pandemic, which has infected almost 10,000 people there and killed 26.
As of Wednesday, it had infected 98 people in Turkey, one of whom died.