A UN humanitarian group on Saturday said it was frustrated by its inability to deliver badly needed aid to the divided Syrian city of Aleppo, amid growing fears of the collapse of a US-Russian truce that went into effect in Syria this week.

"As of Saturday morning, there has been no progress, which is immensely frustrating for the humanitarian community on the ground," said David Swanson, an official at the UN's regional humanitarian affairs office for the Syria crisis.

Dozens of trucks with UN relief supplies remain stuck on the Turkish border. Around 80,000 people will benefit from this aid targeting besieged opposition-controlled districts in eastern Aleppo, Swanson said.

UN envoy for Syria Staffan de Mistura has put most of blame on Syrian President Bashar al-Assad's government, which has not provided any so-called facilitation letters for the aid trucks to cross front lines.

Aleppo, Syria's pre-war commercial hub, has been divided between al-Assad's forces in the west and rebels to the east since fighting erupted for the control of the city in mid-2012.

There have been increasing reports of breaches of the truce since it came into effect on Monday.

Russia, a key ally of al-Assad, accused the US on Saturday of not doing their part to implement the agreements in Syria's ceasefire.

"If this continues, all responsibility for the failing ceasefire in Syria will be with Washington," representative of the Russian General Staff Viktor Poznihir said, according to state news agency TASS.

"Following the fifth day of ceasefire, we state that only the Russian and Syrian sides observed fully their obligations," he told a briefing.

He added that the US and the "so-called moderate groups" it backs have not observed a single obligation under the agreements sealed in Geneva earlier this month.

Rami Abdel-Rahman, the head of the Syrian Obsrvatory for Human Rights, a monitoring group, expressed fears for the truce.

"During the first three days of the ceasefire, breaches were contained. But since Thursday, violations have increased and intensified.I have fears that this will eventually lead to a total collapse of the fragile ceasefire."

Abdel-Rahman told dpa that Islamist rebels, including al-Qaeda affiliates, were bringing in military reinforcements to the edges of Hama in central Syria for a planned massive offensive on regime-held areas there.

The outskirts of Hama saw intense fighting between al-Assad's forces and rebels in the days before the ceasefire that excludes the Islamic State extremist militia and al-Qaeda-linked extremists.

The current truce is the second this year to be brokered by Washington and Moscow. The first, which went into effect in February, failed to hold for long.

More than 300,000 people have been killed in Syria's conflict that started in March 2011, the Observatory estimated this week. 

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