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-Assad of Syria.