UN deputy spokesman Farhan Haq said an understanding was reached on reopening the Bab Al Hawa crossing after talks between UN humanitarian chief Martin Griffiths and Syrian officials.
The crossing from Turkey was a lifeline for about four million people living in the Idlib region, accounting for 85 per cent of aid deliveries. It was closed in July following the UN Security Council's failure to adopt either of two rival resolutions to authorise further deliveries.
Mr Haq announced earlier on Tuesday that Syria had agreed to keep two other crossings to the north-west, Bab Al Salameh and Al Rai, open for another three months, until November 13.
Syrian President Bashar Al Assad opened the two additional crossing points from Turkey to increase the flow of assistance to victims of the magnitude 7.8 earthquake that ravaged north-west Syria and southern Turkey in February. In May he extended their operation for three months, until August 13.
Many of Idlib's inhabitants have been forced from their homes during the course of Syria's 12-year civil war. Hundreds of thousands live in tent settlements and rely on aid that comes through Bab Al Hawa.
Mr Haq gave no details on the agreement reached to reopen the crossing.
The UN's humanitarian office had largely rejected Syria's conditions for the renewal of deliveries through Bab Al Hawa. Damascus has insisted the deliveries must be done “in full co-operation and co-ordination with the government”, that the UN would not communicate with “terrorist organisations” and their affiliates, and that the International Committee of the Red Cross and the Syrian Arab Red Crescent would run aid operations.
The UN responded that the prohibition on communicating with groups considered “terrorists” by the Syrian government would prevent the UN and partner organisations from engaging “with relevant state and non-state parties as operationally necessary to carry out safe and unimpeded humanitarian operations”.
It said stipulating that aid deliveries must be overseen by the Red Cross or Red Crescent was “neither consistent with the independence of the United Nations nor practical”, since those organisations “are not present in north-west Syria”.
The International Rescue Committee (IRC) said on Wednesday that the six-month agreement presented critical challenges and has advocated for a 12-month UN Security Council authorisation to ensure "reliable and scalable" assistance.
"As the IRC we are concerned that the removal of certainty and security provided by Security Council authorisation will impact the ability of humanitarian organisations, and particularly Syrian NGOs, to operate effectively," the group said in a statement.
"The expiry of this agreement in February 2024, at the height of next year's winter season, raises significant concerns about the ability of the response to scale up to meet needs given the lack of predictability.
"A six-month agreement raises critical challenges for hiring and retaining staff, procuring supplies and delivering services that require much longer than a six-month guarantee to implement," the statement said.
The IRC said humanitarian needs in Syria are now at an all-time high, and therefore unimpeded humanitarian access to communities in need by the most direct and timely means possible is of paramount importance.
"This is why the IRC have consistently advocated for a 12-month authorisation of cross-border assistance by the UN Security Council," they said.
Several international organisations had expressed fear that allowing Damascus control over the flow of aid to rebel-held areas could result in limited access to those most in need.