Islamic State group fighters have re-entered Palmyra, nine months after losing the ancient Syrian desert city, activists say.
Meanwhile, Russian-backed Syrian government forces are closing in on the remaining rebel-held area of the city of Aleppo. Civilians are said to be streaming out of the city in large numbers.
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US Secretary of State John Kerry, speaking after a meeting in Paris of governments that back the Syrian opposition, urged Syria and Russia to "show a little grace" as they neared their objective.
Lightning offensive
Syrian government forces were backed by the Russian military when they recaptured Palmyra and its famed ancient Roman ruins from IS earlier this year.The two militaries have since turned their attention to fighting local opposition forces in Aleppo and Damascus, Associated Press reports.
Regrouping in the shadows, by Tomos Morgan, BBC News, Beirut
Even though Islamic State was driven out of Palmyra earlier this year - it's understood they never completely left the area. They were regrouping in the shadows, waiting, for the perfect opportunity to strike.And, whilst the eyes of the world, and the might of Syrian pro-government forces, were focused on Aleppo, they struck.
Palmyra has been seen as a strategic location for IS, due to its close proximity to oil fields. Syrian government reinforcements were sent from Aleppo to fight in this new battle earlier this week.
Rebels groups in the country's second city claim that any more government fighters moving south would hinder President Bashar al-Assad's efforts in Aleppo.
Just as it appeared one fight was nearing its end for Mr Assad, another has only just begun.