LONDON — Syrian and Russian warplanes carried out dozens of airstrikes on Syria’s last rebel-held province on Tuesday, apparently ignoring a warning from President Trump and raising concerns that a large offensive against the densely populated area had begun.

The strikes on Idlib Province, in northwestern Syria along the border with Turkey, came as diplomats scrambled to avert what a United Nations official said could be a blood bath.

Syria’s president, Bashar al-Assad, has vowed to fight until he regains control of the entire country, and his Russian and Iranian allies have been hinting that the battle for Idlib was imminent. About three million people live in the province, including more than a million who were displaced from elsewhere in Syria during the seven-year war.

The province also has an array of rebel forces including jihadists associated with Al Qaeda, which Syria and its allies say they must get rid of.