U.S. President Donald Trump and German Chancellor Angela Merkel agreed in a telephone call on Thursday that a U.N. Security Council resolution calling for a 30-day ceasefire in Syria must be implemented immediately.

"Both agreed that the Syrian regime and its Russian and Iranian allies should promptly and fully implement U.N. Security Council Resolution 2401 (2018). This resolution demands an immediate ceasefire in Syria," Merkel's spokesman said on Friday.

"The chancellor and the president are also concerned about Russian President Putin's latest remarks about arms developments and their negative impact on international arms control efforts," spokesman Steffen Seibert added in a statement.