We investigate a class of generalized Schr\"{o}dinger operators in
$L^2(\mathbb{R}^3)$ with a singular interaction supported by a smooth curve
$\Gamma$. We find a strong-coupling asymptotic expansion of the discrete
spectrum in case when $\Gamma$ is a loop or an infinite bent curve which is
asymptotically straight. It is given in terms of an auxiliary one-dimensional
Schr\"{o}dinger operator with a potential determined by the curvature of
$\Gamma$. In the same way we obtain an asymptotics of spectral bands for a
periodic curve. In particular, the spectrum is shown to have open gaps in this
case if $\Gamma$ is not a straight line and the singular interaction is strong
enough.