German Chancellor Angela Merkel and US President Donald Trump share concerns about developments in Syria, especially the humanitarian situation in the region around Idlib, Merkel's spokesman said after the two leaders spoke by telephone.

"Russia is called upon to act in a moderating manner on the Syrian government and prevent a further escalation," Steffen Seibert said in a statement on Monday evening.

Seibert said the two leaders also spoke about Ukraine, the Western Balkans and trade, he said, without elaborating.

The government of Syrian President Bashar al-Assad, backed by Moscow, says it aims to recapture the rebel-held enclave of Idlib in northern Syria, a refugee for civilians and rebels displaced from other areas of Syria, as well as jihadist forces.

The region has been hit by a wave of air strikes and shelling this month in a possible prelude to a full-scale government offensive.

Merkel had also raised concerns about the situation in Idlib during a meeting with Russian President Vladimir Putin just over a week ago.

Turkey's foreign minister on Friday said it would be disastrous to seek a military solution in the region. Turkey, which has backed some rebel groups there, is trying to avert an attack by forces loyal to al-Assad.