The first six of 40 lorries carrying wheat flour and medical supplies entered Madaya on Thursday evening.
The UN says 40,000 people are living in appalling conditions in
Madaya, which is surrounded by government forces.
A smaller aid convoy also reached two towns under siege by rebels
in the country's north-west on Thursday.
The situation in Foah and Kefraya in Idlib province, where 20,000
people are trapped, is said to be similarly dire.
The UN, International Committee of the Red Cross (ICRC) and Syrian
Arab Red Crescent lorries reached Madaya on Thursday afternoon,
carrying 120 tonnes of flour, as well as medical supplies, blankets
The convoy also includes a nutritionist and health teams to assess
Madaya's residents, between 300 and 400 of whom are said to be
"We are encouraged that we have been able to reach these
towns, where thousands of people have been trapped for very long
periods of time," the UN humanitarian co-ordinator for Syria,
Yacoub El Hillo, told reporters.
"We do not want to see this as a one-off," he added.
"Ultimately the real solution to this predicament, to the plight
of the people besieged in these towns, is for the siege to be
Mr El Hillo added that the World Health Organisation was in direct
talks with the Syrian government to secure the evacuation of hundreds
of people in Madaya in need of urgent medical treatment.
Madaya, in the mountains 25km (15 miles) north-west of Damascus,
has been besieged for six months by government forces and allied
fighters from Lebanon's Hezbollah movement.
"The scenes we witnessed in Madaya were truly heartbreaking.
People are desperate. Food is in extremely short supply," said
Marianne Gasser, head of the ICRC's delegation in Syria, who
accompanied the first aid convoy on Monday.
"The conditions are some of the worst that I have witnessed
in my five years in the country. This cannot go on," she added.