German Foreign Minister Frank-Walter Steinmeier said Wednesday he backed Iranian involvement in a conference in Switzerland this month on ending the brutal nearly three-year-old war in Syria.

"My position is that we should try to include the neighbours -- Iran too," he said after talks in Berlin with his Swedish counterpart Carl Bildt.

UN leader Ban Ki-moon did not include Iran in his invitations to 30 countries to the gathering, due to open on January 22.

And the United States on Tuesday renewed calls for Iran to state that it would support a process in which embattled Syrian President Bashar al-Assad would ultimately step down.

Iran, run by Shiite clerics, considers Assad its main Arab ally. Assad, a member of the heterodox Alawite sect, is fighting Sunni rebels who enjoy support from Gulf Arab kingdoms.

US Secretary of State John Kerry has suggested Iran could play a role in the Syria conference from the "sidelines".

Tehran has scoffed at the suggestion, saying it would only accept offers that respect the country's "honour".

Steinmeier is to join a meeting of ministers from the "Friends of Syria" in Paris Sunday with leaders of the mainstream opposition to Assad ahead of the Swiss talks.

Members of Syria's exiled opposition said Wednesday they had postponed until next week a decision on whether to take part in the peace talks.