It sounds like what you want is... a nuclear device. Your specs aren't all that unusual. From Wikipedia:
Eleven years later, implosion designs had advanced sufficiently that the 5-foot (1.5 m)-diameter sphere of Fat Man had been reduced to a 1-foot (0.30 m)-diameter cylinder 2 feet (0.61 m) long, the Swan device.
Swan devices are recorded as having an explosive capability of 15kT, which is the same as the Little Boy bomb dropped on Hiroshima.
Now if you wanted to actually fire this morter style, rather than giving it a rocket motor, you might have to worry about the ridiculous forces involved, but since we can put a GPS guided computer on board the M982 Excalibur artillery round, I bet we can make something work.
If you want to get smaller, you're going to run into a classification issue. The nuclear powers tend not to like to broadcast their miniaturization capabilities. However, Wikipedia's page on Suitcase Nukes points out that the US has at least one acknowledged small nuke:
The lightest nuclear warhead ever acknowledged to have been manufactured by the U.S. is the W54, which was used in both the Davy Crockett 120 mm recoilless rifle-launched warhead and the backpack-carried version called the Mk-54 SADM (Special Atomic Demolition Munition). The bare warhead package was an 11 in by 16 in (28 cm by 41 cm) cylinder that weighed 51 lbs (23 kg). It was, however, small enough to fit in a footlocker-sized container.
The W54 had a yield of maybe 10-20 tons, according to its Wikipedia article.
If you need something smaller than that, unfortunately, there is literally no more information available to the public. Your guess is as good as mine.