US Ambassador Nikki Haley urged the UN Security Council on Thursday to ratchet up pressure on Russia to end sieges in Syria and help advance peace talks.
"All eyes and all pressure now need to go to Russia, because they are the ones who could stop this if they wanted to," Haley told a council meeting on the worsening humanitarian crisis in Syria.
Haley argued that years of appeals to Bashar al-Assad to allow aid deliveries to reach civilians in besieged areas had failed and that the focus must now squarely be on the Syrian president's main ally.
"Who's the one member-state who continues to protect the regime that is keeping this humanitarian assistance from going through?" asked Haley.
"Russia must live up to its promise to deliver real peace talks and a real political solution," said the US envoy.
"The sieges of the Syrian people must be lifted. The war profiteering and the theft of medical supplies must be stopped."
The council heard a report from UN aid chief Stephen O'Brien who said the humanitarian crisis was worsening in Syria, now in its seventh year of war.
O'Brien called for a pause in fighting in eastern Ghouta, outside Damascus, where he said some 400,000 civilians remain trapped, under siege by government forces.
UN aid convoys have been unable to reach eastern Ghouta since October.
Russia's envoy challenged the UN assessment of the humanitarian crisis, saying a cessation of hostilities was holding "in Syria, as a whole" and that access to besieged areas required "painstaking work."
Petr Iliichev, the Russian charge d'affaires, stressed that Russia, Turkey and Iran were working to uphold a ceasefire.
"Neither you nor your western colleagues have said a word about how you are putting pressure to bear," Iliichev told Haley.
Describing the humanitarian crisis in Syria as a "catastrophe," French Ambassador Francois Delattre said a ceasefire and access for aid deliveries were needed to pave the way to credible peace talks.
The United Nations hopes to bring the Syrian government and the opposition back to Geneva next month for a new round of talks on ending the war despite little progress in previous negotiations.
Delattre said political talks can succeed with "much stronger pressure from Russia on the Syrian regime and a clear political re-engagement of the United States."
More than 320,000 people have been killed in Syria's six-year conflict, which has drawn in Russia and Iran in support of the government, while Turkey, Western countries and Gulf states have backed opposition forces.