Madaya, near the Lebanese border, has been blockaded by pro-Syrian government forces for the past six months.
And the Shiite villages of al-Foua and Kefraya, in the northwest of the country, have been under siege by rebels seeking to oust President Bashar al-Assad.
The Red Cross says food and medical supplies have reached the towns as part of an agreement between the warring sides in Syria.
Desperately-needed humanitarian aid has finally arrived in Syrian towns where people are reported to have died of starvation.
Trucks with food and medicine left the Syrian capital, Damascus, bound for Madaya, near the Lebanese border, under a deal reached with the government.
Thousands of people have been trapped there for months by a government blockade.
The Red Cross says supplies have also been delivered to the pro-government villages of al Foua and Kefraya in the northwestern province of Idlib.
The United Nations World Food Program says activists have reported several deaths from starvation over the past weeks in the affected areas.
WFP spokeswoman Jane Howard says getting the convoy to the towns proved a challenge.
"We've tried again and again to get permission from the different parties on the ground. We've actually put in at least ten official requests since December but food is being used as a weapon of war. This is a siege and people are deliberately blocking and in some places it is one armed group, in other areas it's another, or it's the actual, you know, authorities themselves. And so we managed to a fairly complicated agreement, so that if they allow us to take food into Madaya we'll also take food into other towns in the north of Syria, near Idlib. So these sort of agreements take a long time to organise."
The World Food Program says it's sending one month's worth of food for more than 40,000 people to Madaya, and enough for 20,000 people to al-Foua and Kefraya.
US Secretary of State John Kerry says the United States is continuing to push for regular humanitarian access to the towns.
"We need full access, and that's what we agreed on in the ISSG meeting in New York and in the UN Security Council. And we're raising it with all parties right now, we're pressing for it very hard. It has to happen."
Syria's Ambassador to the United Nations, Bashar Al-Jaafari, insists his country will cooperate with the UN in safely delivering humanitarian assistance to besieged areas.
Mr Al-Jaafari denies the Syrian government is blocking civilian access to restive regions.
He says some of the humanitarian aid previously sent to Syria was looted by opposition and terrorist groups operating in the country.
And Mr Al-Jaafari rejects as fabrications the widespread allegations of starvation.
"There is a problem, yes, but the problem is this: the terrorists are stealing the humanitarian assistance from the Syrian Red Crescent as well as from the United Nations and they are keeping this assistance in their warehouses and then they use it as a leverage of political and financial gain for them to survive. The Syrian government did not stop any convoys of humanitarian assistance. So the Syrian government is not and will not exert any policy of starvation against its own people."
In a separate development, activists have claimed at least 12 children have been killed in an air strike on a school in the rebel-held town of Ain Jara, in Aleppo province.
The coordinator of the Syrian opposition, Riad Hijab, says Russian planes carried out the bombing.
Mr Hijab has cast doubt on the possibility of scheduled peace talks going ahead with the Syrian government.
After a meeting with French President Francois Hollande, Mr Hijab says Russia and the international community must end attacks on Syrians.
"We want to go to these negotiations, we want to negotiate, we are serious about it but to do that the conditions have to be there. We cannot negotiate with the regime when there are foreign forces bombing the Syrian people."
France's foreign minister, Laurent Fabuis, says Syria and Russia must stop military operations against civilians.
"France solemnly calls for the immediate and effective lifting of sieges of all those cities for an unlimited humanitarian access and a stop to the indiscriminate attacks against civilians. I have asked our permanent United Nations representative to consult the Security Council on this. I call on the regime's responsibility, only weeks away from the start of the intra-Syrian negotiations with the opposition."
The Syrian government and opposition are due to meet in Geneva for peace talks on January 25.
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