WASHINGTON — In what would mark a dramatic shift of policy from the international diplomatic effort it has pursued the past year, the United States warned Russia Sept. 28 that it will suspend US-Russian engagement on Syria if Russia does not take immediate steps to halt a Russian-backed Syrian regime assault on rebel-held eastern Aleppo that has killed 96 children since Sept. 23, according to United Nations agencies.

The warning, made in a phone call by Secretary of State John Kerry to his Russian counterpart Sergey Lavrov Sept. 28, comes as the State Department said nondiplomatic options are being discussed for dealing with the mounting loss of life in the Russian and Syrian air assaults on Aleppo. But US officials stressed they still consider a diplomatic solution that would restore the Syria cease-fire deal and get the Syrian parties into political talks in Geneva their preferred option.

“Moscow has a decision to make,” State Department spokesman John Kirby told journalists at the State Department press briefing Sept. 28.

“Given what we have seen in the ground and in the air, around Aleppo, unless something dramatically changes very, very soon … we are going to have to take those steps to suspend our bilateral engagement on Syria,” Kirby said. “That is not an insignificant move for us.”