A young Syrian girl is among the many in Aleppo bidding farewell to the world through social media, as Syrian military forces lay claim to the last rebel neighbourhoods in the besieged city.

Seven-year-old Bana Alabed, who captured international attention with her tweets about life during the four-year siege, tweeted a series of goodbyes late Monday and early Tuesday.

"This is my last moment to either live or die," the girl tweeted early Tuesday, in the last message posted on her Twitter feed. Earlier messages signed by the girl or by her mother, Fatemah, indicate the family was in imminent danger. "My dad is injured now, I am crying," the girl tweeted on Monday.

"Final message – people are dying since last night. I am very surprised I am tweeting now and still alive," Fatemah Alabed wrote in another tweet from Monday.

The Syrian government says it has seized 99 per cent of the neighbourhoods formerly held by rebels in the city, which has been torn apart during the four-year conflict.

The UN's human rights office reported on Wednesday that Syrian forces have killed at least 82 civilians in four rebel-held neighbourhoods. Eleven women and 13 children are among the dead, according to the report.

The International Committee of the Red Cross issued a on Tuesday, calling for all sides to spare the lives of those in eastern Aleppo, where the last neighbourhoods of rebels remain.

Oxfam also encouraged mercy on all sides. "There must be no reprisal attacks, arbitrary detention, extrajudicial executions or disappearances under any circumstances," Andy Baker, Oxfam's regional program manager for the Syria crisis response, said in a statement.

With Syrian forces closing in on those neighbourhoods, the last few free activists in Aleppo said their goodbyes on social media. Many said they were preparing for execution or jail time.

Abdulkafi Alhamdo, a friend of Bana Alabed's mother, shared a final message through Periscope, in which he says Syrian troops are only 300 metres away. "No place now to go," he says in the video. "At least we know that we were free people… We didn't want anything else but freedom."

Alhamdo blamed Syrian and Russian forces for the fall of Aleppo, and called their attack "the most horrible massacre that is in our history." He also condemned the efforts of the United Nations and the international community. "Don't think that they are not satisfied with what's going on," he said.

Others also shared their final messages online, in which they slammed the UN and other world powers for failing to come to their aid.

With files from The Associated Pres