Boris Johnson has said it is "unacceptable" the United Nations has still not had access to eastern Aleppo, almost a week after a landmark deal was reached.
The Foreign Secretary also renewed his calls for a "genuine transition" in power in Syria, away from what Mr Johnson called the "brutal regime" of President Bashar Assad.
On Monday the UN Security Council adopted a resolution that demanded full access for the UN across Syria, which would also allow the body to monitor evacuations from the war-torn city.
However, the UN informed members of the Security Council on Friday that it has still not had access to eastern Aleppo, where thousands of people had been left stranded after the fighting.
Mr Johnson said: "It is unacceptable that the UN has still not been given access to eastern Aleppo to assist those who remain and carry out monitoring.
"The regime and those who back it must immediately deliver on their obligations and comply with Security Council Resolution 2328 to ensure that civilians are protected."
On Thursday the Syrian regime fully recaptured Syria's largest city, after months of intense fighting.
But the Foreign Secretary called it a "hollow victory" that had been "utterly dependent on foreign militias and Russian air support".
Mr Johnson went on to say Mr Assad had "caused destruction on a tragic scale" and could never unite the country.
Mr Johnson added: "There is only one way to end this conflict: a Syrian-led political settlement based on a transition away from the Assad regime.
"The international community - including Russia and Iran - must now work to support a nationwide cessation of hostilities, to help create the environment for a political settlement based on genuine transition."
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