Kernel density plot on a US map

I am trying to plot the kernel density plot superimposed on a US map. The
data consists of the the latitude and the longitude of a few locations in
the US and a number (such  as count) corresponding to each location. I would
like to generate a kernel density plot with the count as the variable
superimposed on US map so that I can visualize how the count varies across
the US. I have looked at packages such as spatstat about generating the
kernel density plots but unable to superimpose the density plot on the US
map???

Can someone help me on how to do this in R? 

Thank you.

Ravi

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