United States Secretary of State, John Kerry delivers a speech during the 28th meeting of the Parties to the Montreal Protocol in Kigali, Rwanda on October 14, 2016.

With military options all but eliminated, the United States and Britain on Sunday said they were considering new sanctions to pressure the Syrian and Russian governments to halt an offensive against rebel-held parts of Aleppo, Syria's largest city.

While the close allies said diplomacy was their primary focus, the tone was tougher than Saturday's message from U.S. Secretary of State John Kerry after he launched a new diplomatic effort to resolve the 5 1/2-year civil war.