The World Health Organization (WHO) has been unable to get a desperately needed medical aid convoy through to civilians in the rebel-held part of Aleppo despite a government promise last month to give it access.

"Delays often happen due to operational and/or security reasons but details are not to be shared," WHO spokesman Tarik Jašarević said in an email on Tuesday.

In a statement this week, the WHO said 240,000 medical treatments from it and the Syrian Arab Red Crescent were being held in a warehouse in the government-held part of the city, Syria's biggest, "for further distribution to the targeted areas, which will begin shortly".

On Dec. 22, the WHO said it had received a promise to be allowed to deliver aid to rebel-held parts of Aleppo, which it planned to transport within the week, and also to the besieged districts of Mouadamiya, in Damascus, and Eastern Ghouta, outside the capital.

The non-governmental Union of Syrian Medical Relief Organisations, made up of Syrian doctors, says cholera, typhoid, scabies and tuberculosis are spreading among the 360,000 people in rebel-held Aleppo for lack of treatments or vaccines. The area is cut off on three sides by the Syrian army.

All sides in Syria's three-year civil war have prevented medical supplies crossing front lines, fearing they could be used to help wounded enemy fighters.

The WHO says surgical supplies such as syringes and bandages have previously been removed from convoys at checkpoints run by the security forces.

Syrian officials could not be reached for comment on Wednesday or Thursday. Damascus denies blocking aid.

Jašarević said vaccines and syringes had been delivered to Eastern Ghouta, the first "complete package" to that area by the WHO in more than two years. It did not give an update on Mouadamiya.

The United Nations says at least 212,000 people remain besieged, mostly by the government, but also by insurgents.

More than 200,000 people have been killed in Syria's conflict, which began in March 2011 with popular protests against President Bashar al-Assad and spiralled into civil war after a crackdown by security forces.