The UN says a deal has been reached with Syria to reopen the sole crossing for UN aid deliveries to its rebel-held north-west after weeks of impasse.

The future of the crossing had been in doubt after the UN Security Council failed to agree on extending its use.

The route from Turkey is a lifeline to 4.1 million people dependent on aid.

The terms of the deal are unclear, but aid agencies have expressed fear that Syria might now manipulate the distribution of food and medicine.

Syria had previously been excluded from any say over the process. Along with its ally Russia, it had strongly objected to the UN-authorised operations through the Bab al-Hawa transit point, describing them as a violation of its sovereignty.

"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," said global relief group the International Rescue Committee.

The UN said the deal meant the crossing would be open for the next six months. Operations had been suspended one month ago after a dispute between Russia and other security council members charged with authorising their continuation.

Bab al-Hawa has become even more important as a conduit for aid since a devastating earthquake struck north-west Syria in February, killing more than 4,500 people and displacing 50,000 families.

The UN said Syria had also agreed to allow it to continue using, for a further three months, two additional border crossings which had been opened with the consent of the Syrian government following the earthquake. Authorisation to use the two crossings had been due to expire on Sunday.