The EU has no plans to impose sanctions on Russia over the conflict in Syria, foreign policy chief Federica Mogherini said Monday ahead of a leaders' summit this week.

"No, we didn't discuss at all sanctions, and there was no member state asking for additional work on sanctions" against Russia, Mogherini said after a meeting of European Union foreign ministers.

EU leaders backed down from threatening sanctions on Moscow over the carnage in Aleppo at their last summit in October amid deep divisions in the bloc about how tough to be on Russia.

But with Russian-backed Syrian forces on the verge of retaking Aleppo from rebels, the EU warned at the weekend it could add to a long list of figures and groups in Syria it has already hit with travel bans and asset freezes.

There had been hints that EU leaders could also act against Russia when they meet on Thursday, especially after the resignation last week of Italian Prime Minister Matteo Renzi, one of the biggest opponents of sanctions.

Mogherini, a former Italian foreign minister, insisted the EU remained united over Syria.

"You find our unity every time we speak," Mogherini told a press conference in Brussels.

"You find the unity of the Europeans not only in statements and our diplomatic action but also in our humanitarian action."

EU leaders are expected at Thursday's summit to back a six-month rollover of damaging economic sanctions against Russia over the conflict in Ukraine.