BEIRUT, Lebanon — The United Nations special envoy to Syria offered unsolicited advice on Monday to President-elect Donald J. Trump: Pay attention to what happens between now and January in Syria, particularly in Aleppo, the storied city at risk of obliteration by Russian-backed Syrian forces.

The envoy, Staffan de Mistura, suggested that Mr. Trump’s shared desire with Russia to collaborate to annihilate the Islamist militants ensconced in eastern Aleppo and elsewhere presented new circumstances that could alter the war — for better or worse.

While such a collaboration could be effective in eradicating the Islamic State, Mr. de Mistura said during an interview in Beirut, the carnage and destruction caused by the Syrian and Russian militaries in Syria increasingly make it more problematic politically for Mr. Trump to align with Russia.

Mr. de Mistura’s remarks, after a weekend of frustrating diplomacy in Syria, represented the first time that he had spoken about Mr. Trump’s potential role in the war through the president-elect’s eagerness to work with Russia, which backs the government of President Bashar al-Assadof Syria.

If Russia desires a new approach with the incoming Trump administration on Syria, Mr. de Mistura said, “it could be difficult for any president in the United States, regardless of his own priorities, to ignore the international outrage” over the “humanitarian tragedy in Aleppo.”

Unless drastic steps are taken, Mr. de Mistura said, “eastern Aleppo will not be there by the new year in terms of structural destruction.”

For now, the Syrian government’s apparent determination to eradicate the anti-government insurgents in eastern Aleppo appears to have overwhelmed all other considerations.

While the bombing of Aleppo abated during Mr. de Mistura’s visit to Damascus on Saturday and Sunday, it resumed as he was departing on Monday. Humanitarian agencies have pleaded for respite from the assaults, which they say have destroyed or incapacitated all hospitals in the rebel-held part of the city.

At the United Nations on Monday, the organization’s top relief official told the Security Council that he was “more or less at my wit’s end as a human being” about the suffering in Aleppo and other besieged parts of Syria.

“We are not just seeing a resumption of violence in Aleppo — this is not business as usual,” said the relief official, Stephen O’Brien, the under secretary general for humanitarian affairs and emergency relief coordinator. “What has been unleashed on civilians this past week is yet another low in an unrelenting inhuman onslaught.”

Mr. de Mistura went to Syria with a short-term proposal to help civilians in Aleppo, which included the evacuation of about 200 wounded people at risk of dying because their injuries cannot be treated in a war zone; the delivery of medical supplies; and food aid for the most vulnerable. The emergency plan would also allow doctors, who have been working around the clock, to rotate out for rest.

The Syrian government rejected all tenets of Mr. de Mistura’s plan except the evacuation of the wounded.

Mr. de Mistura has also proposed asking the fighters who are part of the Nusra Front, the Al Qaeda affiliate operating in Syria, to voluntarily leave east Aleppo to allow aid deliveries for the civilians there.

Syria’s foreign minister, Walid al-Moallem, has rejected that proposal.

Mr. de Mistura nonetheless believes the Russians are sympathetic to his ideas for Aleppo, he said, because they do not want to be seen as helping to destroy the historic city.

In articles quoting Russian sources and disseminated by the Russian-based Sputnik news agency and by a Chinese newspaper, Russia has sent decidedly mixed messages about its intentions.

In the Sputnik articles, the officials criticized the United Nations for having failed to use a pause in the Aleppo bombing this fall to evacuate wounded people. The articles also said traces of chlorine and white phosphorus bombs had been found in attacks on government-held western Aleppo. If true, the claims would suggest rebels are using banned weapons.

Mr. Assad’s forces have also been accused of using banned weapons, including by a United Nations panel of inquiry.

However, China’s People’s Daily newspaper has asserted that Russia is not participating in the bombing, suggesting that as Mr. de Mistura indicated, Russia does not want to be associated with the flattening of Aleppo and untold civilian casualties.

The newspaper also suggested that Mr. Trump’s election victory might factor into what it described as Russia’s reluctance.

“At this stage Russia all the more does not want any trouble with regard to civilian casualties on the Syrian battlefield and other issues that could complicate bilateral relations,” the newspaper said.