With Syria peace prospects in a basic state of collapse, the United States sought to project a toughened posture on Wednesday, conducting military exercises with neighboring Jordan and designating two leaders of Syrian jihadist groups as “global terrorists” — a warning that they should be ostracized by other members of the Western-backed insurgency fighting President Bashar al-Assad.

The developments came a day after the top United Nations diplomat assigned to mediate a peaceful solution to the Syrian conflict, now in its fourth year, quit in frustration, blaming both the Syrian government’s intransigence and the inability of the Security Council to take assertive action to support his efforts and alleviate civilian suffering.

The resignation of the diplomat, Lakhdar Brahimi, was described by Secretary General Ban Ki-moon as a collective failure by parties to the conflict. But Syria’s official news media welcomed Mr. Brahimi’s departure and accused him of bias favoring Mr. Assad’s political opponents.

In a new sign of the conflict’s insidious effects, international refugee officials reported Wednesday that a record 33.3 million people worldwide had now been displaced by war, and that Syria was the leading cause — ahead of new displacements from the Democratic Republic of Congo, Central African Republic, Nigeria, Sudan and South Sudan.