LONDON: Humanitarian access to northwestern Syria could be cut off unless Russia halts its aerial bombardment of routes supplying hundreds of thousands of people with food and water, aid workers warned ahead of an international donor conference on Thursday.

The alarm was sounded by an international aid agency, exclusively to the Thomson Reuters Foundation, as a Syrian government offensive, backed by heavy Russian air strikes, threatened to cut rebel supply lines from Turkey into the northern city of Aleppo earlier this week.

As the fighting raged, efforts were being made by the United Nations in Geneva to get the warring sides to negotiate an end to the conflict that has killed 250,000 people, driven a wave of refugees and empowered Islamic State militants.

But with opposition and government representatives refusing to meet each other, aid agencies said it was vital the issue of humanitarian access and Russian bombings be discussed by government and aid officials at the London funding conference.

European governments and EU institutions in Brussels are seeking to respond to a call by Britain, Germany and Norway, which are hosting the gathering, along with the United Nations and Kuwait, to double humanitarian aid to the region.

UN agencies are appealing for a total of $7.73 billion to cope with Syria's needs this year, with a further $1.2 billion required by regional governments for their own plans to deal with the impact of Syria's conflict.

Civilians in the governorate of Aleppo and in the neighbouring province, Idlib, holding out in what is left of their homes, schools, shops and bakeries, rely on humanitarian aid brought in through two designated crossing points from Turkey.

Idlib has been controlled by opposition forces since 2015, while Aleppo is split between government and rebel-controlled areas.

The aid agency said the two crossing points, Bab al-Hawa and Bab al-Salam, were controlled by opposition forces on the Syrian side of the border.

In an internal report, the aid agency said the Russian air force was still targeting the rebel groups behind frontlines in what might be part of a Syrian government strategy aimed at wresting control of the crossing points from the opposition.

"This would constitute a significant victory for the Syrian government as it would essentially cut opposition-held territory off from Turkey," the aid agency said in the report seen by the Thomson Reuters Foundation.

The charity, which has been working in Syria since the war began five years ago, also said the current level of aerial bombardment was making the route too dangerous for humanitarian convoys and complicating life for civilians.