I have a very hard time beleiving that. Without getting into the details, the physics package that would be required to acheive the preguisite thermal and presure pulse to ignite fusion has to be very powerful. Recently the US has been able to develop a silicon based hyper-explosive that is being used to develop a 'clean' nuke. ( a nuclear device in which fusion fuel [lithium hydride] is directly compressed by a symetrical chemical explosion rather then using a fission device that's child isotopes are what produce radioactive fallout.) Keep in mind that non nuclear exposives powerful enought to produce such energy levels have only been produced in the last 2 years. and are extremly dificult to produce.
Red Mercury is an urban legend. The smallest nuclear device that I consider likley to be developed by the former soviet union would be suit cased sized, and useing reasonable estimates for the amount of fissionable material within the device, would produce a yeild of less then 25< kilotons.
If a nuclear device is unleashed on an American city, we would have no choice but to unleash the world single most affective stratigic force on most of the middle east, as well as likley North Korea, and possibly the stratigic forces of the PRC.
Times like this make you long for the good old days of the cold war when we only had one, fairly rashional enemy, now we have many that are armed with weapons and devices that they can't produce, don't understand, but are eminetly willing to use.