Millions of people living in the rebel-held region, several of whom fled the rule of President Bashar al-Assad, will be affected if the UN fails to resume humanitarian aid there.

The United Nations Office for the Coordination of Humanitarian Affairs (OCHA) on Friday said it was concerned the Syrian government has imposed "unacceptable conditions" on the delivery of humanitarian aid to the northwest of the country.

Aid delivery operations through the crossing between Turkey and Syria has been stalled since Monday, when a 2014 UN deal expired.

Syria announced on Thursday it would give state approval for the UN to deliver the aid via the border crossing between Turkey and Syria,

But the Office for the Coordination of Humanitarian Affairs (OCHA) wrote to the Security Council to say the permission depended on "two unacceptable conditions."

One condition is that the UN should not communicate with any groups that the government has designated as "terrorists." This could make it difficult for the UN to coordinate with some of the aid organizations in the region.

Another condition is that the International Committee of the Red Cross (ICRC) and the Syrian Arab Red Crescent (SARC) should supervise and facilitate the distribution of aid. This could give the government more control over how the aid is distributed.

Despite these concerns, OCHA says that the Syrian government's permission can be a basis for the UN to lawfully conduct cross-border humanitarian operations for a specified duration.

Millions of people living in the rebel-held region, several of whom fled the rule of President Bashar Assad during the brutal civil war, will be affected if the UN fails to resume humanitarian aid there.

The its note to the UNSC, the OCHA stressed on the need to engage with the Syrian government to clarify the added terms for humanitarian work in northwest Syria.

"Any such modalities must not infringe on the impartiality (based on needs alone), neutrality, and independence of the United Nations' humanitarian operations," it said.

"We had pre-positioned a lot of material in the area (northwest Syria) before the deadline. So we do have humanitarian aid in place, but obviously we want to get things going as quickly as we can," U.N. spokesman Stephane Dujarric had said earlier on Friday.