The US warned Russia yesterday (Friday) that it would not move ahead with a plan to target jihadist groups in Syria unless Moscow forced the regime to allow humanitarian aid into the besieged city of Aleppo.
As part of a ceasefire agreement that took root on Monday, the US promised Russia that the two powers would begin combined air strikes if the truce between President Bashar al-Assad and rebel forces held for seven days.
But as well as a reduction in violence the US also demanded that supplies be allowed to reach areas under siege across the country and especially in Aleppo, Syria's second-largest city.
The US said yesterday that Russia was not doing enough to pressure President Assad to allow aid into Aleppo and that Washington would not honour its commitment to joint operations until that happened.
John Kerry, the US secretary of state, conveyed the message to Sergei Lavrov, the Russian foreign minister, in a phone call.
"The secretary made clear that the US will not establish the joint implementation centre with Russia unless and until the agreed terms for humanitarian access are met," said John Kirby, a US state department spokesman.
The impasse threatens to bring down the shaky ceasefire. If Russia feels that the US is not upholding its side of the deal it may give the regime the green light to resume air strikes and trigger a total collapse of the truce.
A convoy of UN trucks has been languishing for days on the Turkish border waiting for permission to head to Aleppo and deliver food.
Staffan de Mistura, the UN special envoy for Syria, said Mr Assad's regime had still not delivered the necessary permission forms. "This is something that is required to happen immediately," he said.