A suicide car bombing outside a pro-regime suburb of Damascus and fighting that followed killed 16 Syrian soldiers, as the UN-Arab League envoy began a regional push for peace talks.
Meanwhile, the United States said children were starving to death in besieged residential areas of the capital and demanded that the regime allow aid convoys in.
State media blamed the early-morning blast at the entrance to the mixed Christian-Druze suburb of Jaramana on Saturday on "terrorists," the regime term for rebels.
The Syrian Observatory for Human Rights said a suicide bomber from the al-Qaeda-linked al-Nusra Front detonated an explosives-packed car at the checkpoint between Jaramana and the rebel-held area of Mleha.
Heavy fighting followed, with rebel mortar fire hitting Jaramana, said the Britain-based Observatory, which relies on activists and medics on the ground.
Observatory chief Rami Abdel Rahman says the southeastern suburb is a key pro-regime area and will be exposed if the rebels overrun the checkpoint.
He said they had almost seized it in he fighting, but that government aircraft had launched four strikes to try to force them back.
The war, which erupted after President Bashar al-Assad launched a bloody crackdown on democracy protests inspired by the Arab Spring uprisings, is believed to have killed more than 115,000 people.
Millions more have been forced to flee the country and hundreds of thousands are trapped by the fighting.
Washington condemned the regime's relentless siege of rebel-held Eastern Ghouta and Moadamiyet al-Sham on the capital's outskirts.
There were "unprecedented reports of children dying of malnutrition-related causes in areas that are only a few miles from Bashar al-Assad's palace in Damascus," said State Department spokeswoman Jen Psaki.
"The regime's deliberate prevention of the delivery of lifesaving humanitarian supplies to thousands of civilians is unconscionable. We call on the Syrian regime to immediately approve relief convoys into these areas," she said.
The call came as UN-Arab League envoy Lakhdar Brahimi was to arrive in Cairo at the start of a regional tour aimed at paving the way for peace talks.
World powers hope to convene a peace conference in Geneva next month, but prospects for it remain unclear, with Syria's opposition divided and due to vote next week on whether to take part.
Assad's government says his departure from office cannot be on the table, while the opposition insists he cannot remain in power.
For months, world powers have been pressing for a negotiated solution, and the renewed push for talks comes after a September deal under which Syria agreed to turn over its chemical arsenal for destruction.
Inspectors from the Organisation for the Prohibition of Chemical Weapons said on Friday they have visited 14 out of more than 20 sites in Syria.
In other developments, nine Lebanese kidnapped by rebels in Syria last year were freed, a move that could lead to the release of two Turkish Airlines pilots whose abduction was linked to their capture.
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