Merkel's spokesman Steffen Seibert said the chancellor, who last week raised the issue of Syria while hosting Russian President Vladimir Putin near Berlin, had told Trump during a telephone call Monday that Russia should exercise its influence.

"Russia is called upon to act in a moderating manner on the Syrian government and prevent a further escalation," said Seibert, referring to Syrian President Bashar Assad, who, having gained Russian military backing in 2015, again controls two-thirds of Syria.

Read more: Fatal blast at Sarmada, Idlib 

Syrian Defense Minister Abdullah Ayoub on Sunday said Assad was determined to recapture Idlib, adding that the US was "looking for a way to stay east of the Euphrates River in its fight against the Islamic State (IS) group."

Ayoub's words came despite a warning on Friday made by Turkish Foreign Minister Mevlut Cavusoglu while visiting Moscow that "a military solution" would cause a "catastrophe" among 3.5 million people crammed into the Idlib region.

Idlib 'multi-faceted'

Turkey has backed several rebel groups in Idlib while operating a dozen military observation posts. Assad's regime holds Idlib's southeastern tip.

Russian Foreign Minister Sergei Lavrov on Friday described the situation in Idlib as "multi-faceted,"  asserting that "terrorists" were hiding behind civilians.

Macron rules out Assad

In parallel remarks Monday, French President Emmanuel Macron ruled out Assad staying in power under a "return to normal," saying this would be a "grotesque error."

Addressing French ambassadors, Macron also warned that Assad was on the verge of creating "another humanitarian crisis in the Idlib region," adding that he expected Russia and Turkey to apply weight.

Tripartite summit, but where?

Turkish state television announced Monday that Turkish President Recep Tayyip Erdogan would next week attend a tripartite summit with Putin and Hassan Rouhani, president of Iran, which is also a major Assad backer.

Private Turkish NTV television said that summit would be held in the northern Iranian city of Tabriz, but did not give a date.

Putin's spokesman Dmitry Peskov said the summit was being prepared but added that "nuances" such as date or venue had not yet been "harmonized."

Erdogan had previously hinted at hosting a summit on Syria in Istanbul on September 7, but without confirmation that Merkel and Macron would attend.

On Saturday, a French Elysee source in Paris said the potential participants had decided that "it was still a bit too early to hold this meeting at summit level."

Iranian media on Monday said that Iranian Defense Minister Amir Hatami had, on a visit to Damascus, signed an agreement for reconstruction in war-torn Syria that provided for a continued presence of Iranian forces.

ipj/bw (Reuters, dpa, AFP)