Syria ceasefire 'first step' to more robust arrangement: U.S. official

"It is a first step in what we envision to be a more complex and robust ceasefire arrangement and de-escalation arrangement in southwest Syria, certainly more complex than ones we have tried in the past," the official said on condition of anonymity.

Further discussions will decide crucial aspects of the ceasefire, including who will monitor its enforcement, the official said.

(Reporting by Yeganeh Torbati and David Alexander; Editing by James Dalgleish)