Syrian government forces have blocked what would have been the first aid convoy to a besieged Damascus suburb for more than three years.
The convoy had been given permission “from all parties” to proceed, the UN and Red Cross said.
About 4,000 people are thought to be trapped in the rebel-held suburb of Darayya.
Earlier this month, the UN said the Syrian government was refusing to allow aid to hundreds of thousands of people.
Government forces shelled parts of Darayya on Thursday, the UK-based monitoring group Syrian Observatory for Human Rights (SOHR) said.
The suburb borders a military airport used by Russian planes in support of the government of President Bashar al-Assad.
Elsewhere in Syria:
- Sixteen senior members of the al-Qaeda-linked al-Nusra Front were killed when an air strike hit a meeting in Idlib province in north-western Syria, SOHR said
- In Hama province in central Syria, al-Nusra Front militants shot dead 19 civilians from the Alawite sect of President Assad, SOHR said. The Alawites are viewed as heretics by jihadist groups.
- Rights group Amnesty International said armed groups including Islamists of Ahrar al-Sham and the Army of Islam may have used chemical weapons in attacks on a mainly Kurdish neighbourhood of Aleppo
- The UN has published satellite images it says show the destruction of approximately 60 shelters at a camp for displaced people in a rebel-held area of Idlib last week
The UN humanitarian co-ordinator for Syria, Jan Egeland, said that the aid convoy to Darayya had been preventing from reaching the “starving” area because it had baby milk on board.
UN spokesman Stephane Dujarric told reporters that the UN and Red Cross had aborted the mission to Darayya “because the convoy was refused entry, due to the medical and nutritional supplies on board”.
The trucks carrying baby milk, vaccines, medical supplies and hygiene kits had been stopped at the last government checkpoint, Mr Dujarric said.
“These conditions, imposed by government security personnel, were unacceptable, and contrary to earlier guarantees and approvals obtained from the Syrian government,” he added.
The convoy was supposed to be a first step towards bringing in more aid for the people in Darayya, ICRC spokeswoman Krista Armstrong said.
“What we were bringing in was not enough for the people there. This was supposed to be a first step. We needed to assess what was needed and return,” she said.
She said people in Darayya were living in desperate conditions.
“There’s no electricity or water for the last three years so people have been improvising, burning plastic sort of as an alternative fuel to boil water that’s not salubrious and for other means. That in itself is creating an extremely polluting environment and can lead to respiratory problems,” she said.
The UN says it still does not have government permission to send aid convoys to about half of the 900,000 people in Syria it wants to reach.