French control Montreal, Fort Frontenac and Fort Niagara.
British control Oswego and starts a siege at Fort Frontenac.
Can French use the Fort Niagara card?
I was sure that a city under a siege breaks the connection for the purpose of playing cards, but I cannot find it anywhere.
No, I wouldn't say it breakes controll, since it doesn't say so anywhere in the rules. You need to actually succeed in the siege to get controll out of the french players grip in this scenario.