Amos was presenting the fourth UN report on humanitarian access in war-torn Syria, and stressed that 10.8 million Syrians were in desperate need of humanitarian aid but that workers were having trouble reaching about half of them.
"The focus of the government of Syria remains on controlling the work of the UN and its partners. Four years into this war, we are unable to sustainably reach nearly half of those identified as being in the direst need," Amos said.
Amos repeated her demand that Damascus allow aid to pass into Syria from neighboring Turkey, Iraq and Jordan, which she said would help 1.3 million people.
Syria opposes the delivery of cross-border aid that would send supplies directly to areas held by the armed opposition in its brutal civil war.
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Security Council members have been discussing for weeks a draft resolution that would enforce humanitarian aid deliveries across borders into Syria.
Russia, president of the council in June, said it had proposed an "elegant innovative formula," which was rejected by its Western partners.
Syria has warned the Council that it would consider any attempt by humanitarian convoys to cross the border without its consent an attack.