They'd probably not use conventional strategic nuclear missiles... but rather,
Tactical Nuclear Weapons. (note the red tip is nuclear).
A string of calls to consider the use of TNWs in Afghanistan following the September 11, 2001 terrorist attacks against the United States focused attention on proposals, many of which had been publicized years earlier, to develop new, low-yield nuclear weapons for a limited range of military contingencies--in particular, to destroy deeply buried, hardened bunkers (caves, in the case of Afghanistan). During a briefing on the 2002 Nuclear Posture Review on January 9, 2002, Assistant Secretary of Defense J. D. Crouch told that ?no recommendations? had been made in the report with regard to development of a new type of nuclear weapons, adding, ?We are trying to look at a number of initiatives,? one of which ?would be to modify an existing weapon to give it greater capability against deep or hard targets.?
Low-yield TNWs are sometimes seen as less destructive and thus more usable than other classes of nuclear weapons.
TNW yields range from relatively low (0.1 kiloton (KT)) to higher than the bombs dropped on Hiroshima and Nagasaki (10-15 KT, upwards to 1 megaton). Even a very low-yield atomic blast would create highly damaging effects, above and beyond what a conventional explosion of the same size could produce.