One aid convoy reached besieged Syrian territories in August

Just one aid convoy has made it into besieged Syrian territories in the past the month, the UN has said.

The overall situation of humanitarian access to Syria's besieged areas is "wholly unacceptable", the group said on Monday, as the UN comes under mounting criticism for its seemingly ineffective role in Syria.

The United Nations humanitarian office (OCHA) said a convoy with life-saving supplies finished its delivery on Thursday to al-Waer, a besieged area of the district of Homs.

"While we welcome yesterday's convoy, the level of access to besieged areas this month is wholly unacceptable," an OCHA statement said.

The two-part delivery to al-Waer - which included food and medical items - was "the first full completed" convoy to reach a besieged area in August, OCHA further said.  

The initial al-Waer delivery happened on Tuesday. A total of 75,000 people were reached over the two days.

Top UN officials including envoy Staffan de Mistura have blasted Syria's warring parties in recent weeks for blocking civilians from accessing aid.

Much of the blame has been directed at Syrian President Bashar al-Assad's forces, who are responsible for the vast majority of the sieges of the country's 18 besieged areas.

Meanwhile, de Mistura joined Friday's meeting at a Geneva luxury hotel with US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov.

The talks were expected to centre heavily on the UN's push to restart Syria peace talks.

The UN's outgoing humanitarian coordinator for Syria, Yacoub El Hillo, said it was "a good thing" that the US and Russia were meeting but accused world powers of shying away from such urgently-needed diplomacy for far too long.

Asked in a BBC interview aired Thursday what could have helped avoid a humanitarian catastrophe in Syria, El Hillo said: "Political guts and courage on the part of the international community, including the Security Council."