Rome, Italy: Top EU and UN diplomats warned on Saturday that there could be no victory in the battle for the Syrian city of Aleppo without negotiations aimed at ensuring a viable future for the war-torn country.

"You can win a war but you can lose the peace," said Federica Mogherini, the European Union's foreign affairs chief, at a conference on the Mediterranean region in Rome.

"Who is interested in winning a war in Syria and getting at a price a country that is divided, armed, full of terrorists... isolated in the international community?" Mogherini asked, adding that she did not consider President Bashar al-Assad's regime as having already won the Aleppo battle.

As of Saturday the Syrian army controlled more than half the rebel part of Aleppo after seizing overnight another sector in an offensive that has claimed more than 300 civilian lives and forced tens of thousands to flee the fighting.

UN envoy to Syria Staffan de Mistura, who was also at the Rome conference, voiced concern about the Assad regime's advances in Aleppo.

"If this is going to be an occasion for the government to say: we won the war, and therefore no need for negotiations, I hope not,"  he said, adding that's why he counts on "the influence of Russia and Iran" to convince Damascus to seek a negotiated solution to the conflict.

The loss of Syria's second city to Assad's forces would be the biggest blow yet to Syria's opposition in the more than five-year-old war.

"Now it's time for negotiation, but negotiating in real terms, which means power sharing... Otherwise, the alternative could be no major conflict but a creeping, ongoing guerilla (war) and no reconstruction," de Mistura said. 

More than 300,000 people have been killed since the Syrian conflict started with anti-government protests in March 2011, and over half the country's population has been displaced.