Renewed violence, despite a ceasefire brokered by Russia and Turkey, casts further doubt on UN-led peacemaking efforts.

Fighting in and around the Syrian capital of Damascus has intensified, after surprise attacks by rebel fighters in northeastern parts of the city.

For much of Syria's six-year war, the capital has been shielded from the destruction unleashed elsewhere in the country, but the latest rebel advances were met with air strikes, shelling and sniper fire by Syrian forces.

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Amid the renewed violence, an estimated 300,000 people have been cut off from humanitarian assistance, with aid groups warning of a looming starvation crisis. A report, published last week by Physicians for Human Rights, found the Syrian government has "deliberately" restricted humanitarian access to besieged populations.

Recent clashes have gripped the Jobar district of Damascus, while rebels have also made advances near Hama.

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"After the failure of political conferences and solutions, the military operation is an urgent necessity," Samer Alaiwi, an official from the Jaish al-Nasr rebel group, said on a rebel social media feed.

Increased fighting in recent weeks, despite a ceasefire brokered in December by Russia and Turkey, casts further doubt on UN-led peacemaking efforts in Geneva, where talks resumed this week after little progress was made in previous rounds.