The aid increase, announced by the US State Department on Tuesday, brings the total US humanitarian spending for Syria to about $5.9 billion since the conflict began in 2011.

The money has been earmarked to help the United Nations and independent charities offer food, shelter, safe drinking water and medical care.

The new US commitment was made hours after the World Health Organization (WHO) and the Red Cross called for injured and sick civilians to be evacuated from besieged parts of the northern Syrian city of Aleppo, amid a worsening conflict.

Global health and aid officials said safe corridors needed to be created to allow people to be removed safely.

New areas targeted

The Syrian regime, backed by Russian fighter jets, launched an offensive to retake rebel held areas of the war-ravaged city last week, following the breakdown of the latest ceasefire when an aid convoy was bombed from the air.

The WHO said the city was running short of medical help for an estimated 250,000 civilians, stranded around rebel-held districts of Aleppo.

"All access routes to east Aleppo are closed, leaving no options for the civilian population to leave the besieged area," the WHO's statement said.

"The remaining 25 health facilities in east Aleppo city are on the verge of complete destruction, including seven hospitals that are fully or only partly functioning," it added.

At the same time, a Syrian military official vowed that the latest regime offensive would continue until insurgents are "wiped out."

Rising death toll

Thirteen more civilians were killed in air strikes in the eastern part of the city on Tuesday, rescue workers said, bringing the death toll since September 19 to 262, including 42 children.

Syrian state TV said its troops captured Farafra, a rebel-held area, close to Aleppo's famous citadel on Tuesday.

The Syrian Observatory for Human Rights monitoring group also confirmed that regime forces had advanced against rebels in the Old City.

"There was intense shelling earlier [in eastern Aleppo.] It seemed the [government] was preparing for the attack," said Ibrahim Alhaj, a member of volunteer first responders also known as the White Helmets, who operate in opposition controlled areas of Syria.

A senior Syrian rebel official told the Reuters news agency that regime forces had attacked rebel-held areas on four fronts during the day.

Germany has called for a temporary no-fly zone for military aircraft over Syria to allow aid agencies to distribute aid.

mm/kms (AP, AFP, dpa, Reuters)