The United Nations and its partners are starting to get basic supplies to besieged areas in Syria but haven't reached about 20 percent of those in urgent need and are "extremely concerned" about some 500,000 people caught behind front lines, according to a joint statement Monday.

U.N. humanitarian chief Stephen O'Brien and the 10 other signatories said some 4.6 million Syrians "are barely existing in places that few can leave and aid cannot reach."

They singled out about 500,000 people who can't be reached because of fighting in Aleppo, Syria's largest city before the war, and northern rural Homs. They also noted that two million people are in areas controlled by the Islamic State extremist group that are not getting aid.

The statement said that since the beginning of the year, the U.N. and its partners have reached over six million people with aid through regular deliveries and recent convoys to some besieged towns. But it said not enough aid is getting through and medical supplies and equipment are still being removed at checkpoints, which is "unacceptable."

U.N. spokesman Stephane Dujarric said Monday that planned convoys to four besieged towns — Madaya, Zabadani, Foua and Kfarya — had to be postponed "due to security considerations."

The joint statement said Syria today is "almost unrecognizable in parts" from the country before the conflict began five years ago, on March 15, 2011. Over 250,000 people have been killed and over half the population has been forced from their homes "out of fear and want," it said. An additional 4.8 million Syrians have fled the country.

The officials expressed hope that the "signs of momentum" and "fragile glimmers of hope" that have led to fewer bombs falling and more humanitarian access in recent weeks will lead to real change.

Those signing the statement included the heads of the U.N. agencies dealing with food, children, refugees, health, development and Palestinian refugees, as well as the U.N. special representatives for children and sexual violence in conflict and the leaders of the International Organization for Migration and InterAction, an alliance of U.S.-based international organizations focusing on relief and development.