A United Nations aid delivery system meant to ensure that help reaches an opposition-held province in northern Syria will expire next month unless the Russians allow it to continue.
Moscow - an ally of Syrian President Bashar al-Assad - wants all aid to be routed through his government.
The US ambassador to the UN visited Syrian refugees living along the Turkey-Syria border to assure them that it will not happen.
The US ambassador to the UN is in southern Turkey, meeting NGOs and Syrian refugees.
Linda Thomas-Greenfield is on a mission to renew cross-border humanitarian assistance.
On July 10, the mandate of a UN mechanism allowing aid to be provided directly through Turkey's borders will expire.
If it is not renewed, UN agencies will have to reroute all their operations through al-Assad's government in Damascus.
Al Jazeera's Zeina Khodr has more.
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