An aid convoy on Wednesday entered two rebel-held towns, besieged by government forces, near the Syrian capital Damascus, for the first time since 2012, the International Committee of the Red Cross (ICRC) said.
The convoy, composed of 37 trucks, is to deliver food and medical assistance to people of Zamalka and Arbeen in the Eastern Ghouta region, ICRC said in a statement.
"We are finally bringing much-needed humanitarian aid to 20,000 persons."
The ICRC said they provided medical items such as hygiene parcels and primary care medicines to treat up to 10,000 patients for three months in both towns, while the UN vehicles carried food parcels and wheat flour, as well as nutritional and medical items.
The UN delivered humanitarian aid to 16 besieged areas in Syria earlier this year, except Zamalka and Arbeen.
Earlier this month, an aid convoy entered the Damascus suburb of Daraya, under siege from government forces since 2012, as an international deadline for relief provisions to beleaguered areas of Syria expired.
The United States, Russia - an ally of the Syrian government - and other countries jointly announced that if humanitarian aid was denied to besieged areas starting from June 1, the UN's World Food Programme would pursue air drops.
UN Syria envoy Staffan de Mistura, who briefed the UN Security Council in New York Wednesday, said that reaching 18 besieged areas was "quite a landmark" compared to previous years.
Despite the somewhat improved aid access, de Mistura said he has not set a date for the next round of Syria peace negotiations, noting that the talks need to be "particularly well prepared" ahead of the August 1 deadline set by the International Syria Support Group to start on a political transition.
"We are aiming [for] within July, but not at any cost and not without some guarantees," he said.
Elsewhere in Syria, Islamic State on Wednesday forced a US-backed rebel group to retreat from territory they had earlier seized near a key town held by the extremist militia on the border with Iraq, a monitoring group reported.
The retreat followed a fierce counter-attack by Islamic State against the rebel group, the New Syria Amy (NSA), near the town of al-Bu Kamal, the Syrian Observatory for Human Rights said.
The militants recaptured al-Hamdan airport, 5 kilometres north-west of al-Bu Kamal, hours after the NSA rebels had taken over the facility, said the Britain-based watchdog, which relies on a network of activists inside Syria.
Al-Bu Kamal is a major link between Islamic State's territories in Syria and Iraq.
The NSA, a small force trained by neighbouring Jordan, retreated to areas outside the eastern province Deir al-Zour near the border with Iraq, after the rebel group had lost at least seven fighters in clashes with militants, the Observatory added.
On Tuesday, the NSA had advanced within 15 kilometres of al-Bu Kamal that lies about 120 kilometres south-east of Deir al-Zour, the main city in the Syrian province of the same name, which is almost entirely controlled by Islamic State.
In northern Syria, at least 10 people were killed in a car bombing in a Kurd-controlled Syrian town near the Turkish border, a local doctor said.
The explosion occurred in the centre of Tel Abyad, also leaving 30 people injured, Saleh al-Shawakh, a doctor at the town's hospital, added.
No one has claimed responsibility for the attack.
In 2015, Kurdish fighters took full control of Tel Abyad from Islamic State.
By seizing the ethnically mixed town, the Kurdish People's Protection Units - a powerful Kurdish militia - cut off Islamic State's most direct supply route, which had run from the Turkish border to the extremist group's Syrian stronghold in al-Raqqa.
The Syrian Kurds are widely seen as the most effective force fighting Islamic State.
The Kurdish-led Democratic Forces of Syria, backed by US-led airstrikes, are currently fighting Islamic State inside the northern Syrian city of Minbij.
In recent months, the hardline jihadist group has been fighting on several fronts in Syria and Iraq.