The UN has relied on costly aid air drops to Qamishli, northeast of Raqa, from government-held Damascus.
"Courageous humanitarian workers have been now travelling for days to open a new corridor from Aleppo in the west to Qamishli," Jan Egeland, head of the UN-backed aid taskforce for Syria, told reporters in Geneva.
"We are hopeful that we will be able to reach that place very soon now, within hours hopefully," he added.
Humanitarian workers have faced daunting challenges in responding to the crisis in Raqa, where the US-backed Syrian Democratic Forces are battling to oust IS.
Tens of thousands of people have fled, but access to the remote region has proved difficult. The UN has relied on costly aid air drops to Qamishli, northeast of Raqa, from government-held Damascus.
Road convoys, which would get more supplies in than expensive air drops, was the only "sensible" option, said Egeland.
The mission expected shortly in Qamishli was a small "reconnaissance convoy", he added.
But the UN has voiced concern about the safety of the route, which had been closed since 2013, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group.
An estimated 300,000 civilians once lived under IS rule in Raqa, including 80,000 displaced from other parts of Syria before the group seized the city.
The UN estimates that nearly 170,000 people fled Raqa city and its environs in April and May alone, and thousands of displaced civilians are now living in overcrowded and under-resourced camps.
IS seized Raqa in 2014, transforming it into the de facto Syrian capital of its self-declared "caliphate".
The UN expects new waves of displacement as the battle inside Raqa city progresses.