  We construct a solution to the low-energy string equations of motion in five
dimensions that describes a circular loop of fundamental string exponentially
expanding in a background electric $H$-field. Euclideanising this gives an
instanton for the creation of a loop of fundamental string in a background
$H$-field, and we calculate the rate of nucleation. Solutions describing
magnetically charged strings and $p$-branes, where the gauge field comes from
Kaluza-Klein reduction on a circle, are also constructed. It is known that a
magnetic flux tube in four (reduced) spacetime dimensions is unstable to the
pair creation of Kaluza-Klein monopoles. We show that in $(4+p)$ dimensions,
magnetic $(p+1)$ ``fluxbranes" are unstable to the nucleation of a magnetically
charged spherical $p$-brane. In ten dimensions the instanton describes the
nucleation of a Ramond-Ramond magnetically charged six-brane in type IIA string
theory. We also find static solutions describing spherical charged $p$-branes
or fundamental strings held in unstable equilibrium in appropriate background
fields. Instabilities of intersecting magnetic fluxbranes are also discussed.
