Syria's air force on Saturday struck a besieged, rebel-held town near Damascus where UN and Red Crescent workers distributed aid, a monitoring group and activists said.
"Two air strikes hit Douma during a visit of a delegation of the United Nations to the town's outskirts," said the Syrian Observatory for Human Rights.
Activists said aid distribution went ahead even though one of the strikes hit an area near a warehouse being used to store relief supplies.
Douma is in the Eastern Ghouta area of Damascus province, where local residents suffer food and medical shortages.
Activist Hassan Takieddin said 400 aid parcels were distributed "to the whole of Eastern Ghouta. That is very little."
The UN's visit to the town was the first since March, he said.
A UN Security Council resolution was passed in February demanding aid access to people in need across Syria.
Rights groups and the United Nations have both said aid was still not reaching millions of people in need, despite the resolution.
Meanwhile, state news agency SANA said four people were killed and nine wounded in a rebel bomb attack in eastern Damascus.
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Thirty kilograms (65 pounds) of explosives were used in the blast, a police source told SANA.
Meanwhile, the army and rebels were holding ceasefire talks in Qadam and Assali, both in southern Damascus, and the Waar district of the central city of Homs, the Observatory said.
In Aleppo, troops evacuated wounded comrades and detainees from the central prison that had been under rebel siege for more than a year, said the Observatory.
The army, backed by pro-regime militiamen and Lebanon's Shiite movement Hezbollah, broke the rebels' siege of the prison on Thursday.
Elsewhere, the Observatory said the death toll from a Thursday evening attack by rebels on an election rally in Daraa for President Bashar al-Assad had risen to 37.
"Among them were 19 civilians, including four children, 12 members of the popular defence committees (pro-regime militia) and six soldiers," Observatory head Rami Abdel Rahman told AFP.
Activists on Saturday reported 13 barrel bomb strikes by regime aircraft on Nawa, a rebel bastion in Daraa province of southern Syria.
Assad, facing two little-known challengers in a June 3 presidential election, is widely expected to clinch a third seven-year term despite Syria's civil war, which has killed more than 160,000 people.
The election will only be held in regime-controlled areas, and has been dismissed by the opposition and its Western backers.