The aid deliveries from Turkey via the Bab al-Hawa crossing stopped in July when Western powers and the Syrian government's main ally Russia failed to agree on extending a U.N. Security Council mandate for the operation. Syria then gave unilateral approval - but on terms which the U.N. rejected as unacceptable.

After weeks of diplomacy, a Syrian government letter sent to the United Nations this week and seen by Reuters did not mention the rejected conditions. U.N. Secretary General Antonio Guterres welcomed an "understanding" with Damascus on using the Bab al-Hawa crossing for six months, his Deputy Spokesman Farhan Haq said on Tuesday.

The northwest is the last major bastion of rebels fighting President Bashar al-Assad in the 12-year-long Syrian war, and millions of people there depend on U.N. aid.

NGOs and individual states have long organised unilateral aid convoys into the northwest, but U.N. agencies will not cross the border without government or Security Council approval.

The U.N. had been using Bab al-Hawa since 2014 with Security Council authorization. Syria rejected the operation as a violation of sovereignty.

The terms set by Damascus in July included barring the U.N. from engaging with what it calls "terrorist organizations" in the rebel-held region. They also limited who could oversee and facilitate deliveries to the Syrian Arab Red Crescent (SARC) and the International Committee of the Red Cross (ICRC).

An Aug. 5 letter sent by U.N. emergency relief coordinator Martin Griffiths to Syria's U.N. Ambassador Bassam Sabbagh and seen by Reuters said the U.N. "may need to engage with different actors in north-west Syria" while it conducted aid operations.

It also said that his office, not the ICRC and SARC, held the mandate to oversee operations in northwest Syria and that the ICRC and SARC did not have sufficient presence in that region to take over such humanitarian work.

In an Aug. 6 letter, Sabbagh thanked Griffiths "for the clarifications on some of the essential operational modalities" and said Syria "looks forward to the involvement" of the ICRC and SARC "when circumstances permit", without mentioning the earlier conditions.

The Syrian government did not immediately respond to questions on whether it had loosened the preconditions and why.

'DEVIL IN THE DETAIL'

The spokesperson for the U.N.'s Guterres said late on Tuesday that Syria had reaffirmed its "consent" in recent days to the crossing's use and that the agreement would allow the U.N. and partners to provide cross-border aid "in a principled manner that allows engagement with all parties".

A spokesperson for the U.N.'s humanitarian operations agency OCHA said the agency was "ready to resume aid operations through the Bab al-Hawa crossing as soon as possible".

Sherine Ibrahim, Turkiye director at CARE International, said the letters "did not mention the previous conditions related to ICRC/SARC or not working with de facto authorities".

But that hadn't fully allayed concerns.

"When we read these letters, we don't see the detail - and as we know the devil is always in the details," she said.

A Damascus-based aid worker told Reuters that Syria's letter indicated it "accepts U.N. principles" and that aid could now resume through Bab al-Hawa without the "new conditions".

The announcement came as Syria extended unilateral permission for two other border crossings from Turkey to be used to bring in aid, which it had initially granted in the wake of the Feb. 6 earthquake.

Tanya Evans, country director at the International Rescue Committee, noted the approval agreed for the use of Bab al-Hawa expires in February, in the depths of winter, raising concerns about the ability to scale up the aid response.

Evans said the IRC preferred the security and stability of a long-term Security Council resolution over the Syrian government's unilateral and short-term approval.

(Reporting by Maya Gebeily; Editing by Tom Perry, Sharon Singleton and Gareth Jones)

By Maya Gebeily