The United Nations has delivered humanitarian aid to 16 of the 18 besieged areas in Syria but two rebel-held areas near Damascus have not received any assistance since November 2012, the U.N. said Monday.

U.N. deputy spokesman Farhan Haq said the U.N. has still not been able to deliver any aid to the Damascus suburb of Zamalka and the town of Arbeen east of the Syrian capital, both under siege by government forces.

Haq said a convoy delivered food, health and nutrition supplies Sunday to two other besieged areas near Damascus, Ein Terma and Hamouria. That convoy also delivered assistance to three hard-to-reach communities in the Kafr Batna area, also near Damascus, which means a total of 25,000 people in five besieged and hard-to-reach areas received aid on Sunday, he said.

Since the beginning of January 2016, nearly 850,000 people in hard-to-reach areas — including more than 330,000 in besieged locations — have received assistance, he said. During that time, 86 convoys have delivered aid compared with a total of 50 in all of 2014 and 34 in 2015, he said.

But Haq said "much more progress is required."

He called for "unconditional, unimpeded and sustained access to the millions of people in besieged and hard-to-reach locations across Syria."

The 5-year-old civil war in Syria has killed some 250,000 people, displaced millions and left vast swaths of the country in ruins, enabling the Islamic State extremist group to take control of large areas of the country. A Russia- and U.S.-brokered truce between the government and some rebel factions began on Feb. 27, but fighting has continued in many areas.

The U.N. has been pressing for unhindered humanitarian access throughout Syria for years, but access by road has been repeatedly rejected by the government of President Bashar Assad and its forces manning checkpoints. Aid deliveries by air remain an option though U.N. officials have repeatedly pointed to the high cost, complexity, security risk and smaller quantities of aid in air drops compared to road deliveries.

The International Syria Support Group, a coalition of world powers seeking to end the Syria conflict, had called for the U.N. World Food Program to unilaterally deliver food to besieged Syrians starting June 1 if access wasn't granted by the government. But any air drops or helicopter deliveries would require a green light from the government.

Diplomats say, however, that pressure from the group appears to have sparked an increase in approvals for land convoys.

With two besieged areas still not reached, he said air drops or airlifts involving helicopters remain a possibility, depending on whether the area is in an urban location.