Jan Egeland, who heads the humanitarian taskforce co-chaired by the United States and Russia, said there has been sustained progress in delivering life-saving supplies.
The United Nations has identified 18 areas in the war-ravaged country it considers to be besieged.
Syrian President Bashar al-Assad's government on Tuesday gave "a verbal greenlight to go to some new besieged areas", meaning the taskforce now has permission to reach a total of 15 locations, Egeland said.
IS-controlled Deir Ezzor, with an estimated 200,000 besieged people, also remains inaccessible but England said plans were being firmed up for a humanitarian air drop.
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"It's a major operation," Egeland told reporters, adding that it would be led by the UN's World Food Programme (WFP) with logistical help from major powers like the EU, Russia and the United States.
A WFP air drop over Deir Ezzor last month faced technical hurdles since it had to be carried out at a very high altitude.
He made the comments in Geneva, where talks to end Syria's five-year civil war were finishing a second week and as a ceasefire declared on February 27 remained broadly in place.
Following the latest greenlights from Damascus to reach new areas, Egeland said plans were on track to get aid to roughly 1.1 million people by the end of April.
On Wednesday, the UN, ICRC and SARC, travelling in a 27-truck convoy, delivered aid to 70,000 people in the Houlah area, which has been under siege for three years, the ICRC said in a statement.
In total, the UN estimates that nearly 4.5 million Syrians are currently living in besieged or hard-to-reach areas.