quantum field theory - Derivation of non-local conserved charges

Consider a 2D sigma model with a symmetry group $G$ and whose generators obey $$[T_A, T_B] = f^C_{AB} T_C$$ and whose conserved currents are Lie algebra-valued, i.e. $$j_{\mu} = j_{\mu}^A T_A$$ and $$\partial_{\mu} j^{\mu A} = 0.$$

From this it is straightforward to construct a conserved quantity $$Q^A = \int_{-\infty}^{\infty} j^{0A} (x,t) dx$$ where $$\dot{Q}^A = 0.$$

In integrable systems, we can generate an infinite amount of additional, non-local conserved charges if the current satisfies the flatness condition $$\partial_{\mu} j_{\nu} - \partial_{\nu} j_{\mu} + [j_{\mu}, j_{\nu} ] = 0 ~,$$ where indices are raised/lowered with a Lorentzian metric. The first of these non-local charges is given as

$$Q^{A~(1)} = f^A_{BC} \int_{-\infty}^{\infty} dx \int_x^{\infty} dy \; j^{0B}(x,t) j^{0C}(y,t) - 2 \int_{-\infty}^{\infty} dx \; j_1^A(x,t) ~.$$

I don't understand how this result is obtained from the flatness condition, and I can't find a derivation anywhere (the result is just stated, e.g. in section 2 of http://arxiv.org/abs/hep-th/0308089 and equations (11-12) of http://arxiv.org/abs/hep-th/0404003).

Any help would be appreciated.