The Russian side is to receive the final instructions from Moscow on Friday.
"We have concerns, we have reservations. We need to study everything carefully because the resolution is complicated with many components," the diplomat said.
"We are supposed to receive the instructions tomorrow," Ilyichev stressed.
Under the US draft resolution distributed on Wednesday, the sanctions will become effective automatically as of September 1, if South Sudanese President Salva Kiir refuses to sign a peace agreement with the rebels and a ceasefire regime fails to come into force.