According to a convenient nuclear effects calculator I found[1], 20MT at 5km and an optimal burst height will generate over 40psi of overpressure. The fireball itself will reach about 4km in radius. According to another site[2], 20psi is enough to severely damage or destroy heavily built concrete buildings, and the maximum wind speed at 40psi will be greater than the speed of sound. For comparison, 3psi is enough to destroy a normal house.
So, yeah, 5km counts as "close range".
[1] http://www.fourmilab.ch/cgi-bin/Bombcalc?yield=20&yunit=...
[2] http://www.atomicarchive.com/Effects/effects4.shtml