ANKARA (Reuters) - Turkish officials have told a visiting Russian delegation that attacks on Turkish observation posts in northwestern Syria must be stopped immediately and that such assaults will not remain unanswered, the Turkish presidency said on Monday.

Earlier, Ankara said Russian-backed Syrian government forces had killed five Turkish soldiers, among thousands deployed there to help stem a Syrian offensive to retake the last rebel bastion in the country after nearly nine years of civil war.

In a statement following talks between Turkish presidential adviser Ibrahim Kalin and the Russian delegation, the presidency said the Russians were told that attacks on Turkish forces were unacceptable and that Moscow must fulfill its duties as mandated by a 2018 de-escalation deal between Ankara and Moscow.