La1-xSrxMn1-yFeyO3 (briefly LSMFO) belongs to a family of hole-doped mixed-valenced (Mn3+/4+) manganite perovskites, attracting considerable attention due to a rich magnetic phase diagram and interesting transport properties, including colossal magnetoresistance [1]. Electronic and magnetic properties of these compounds are influenced strongly by interplay between ordering of the spin, charge and orbital degrees of freedom [1], as well as by the phase separation or generation of nanosize hole-rich ferromagnetic (FM) particles in the paramagnetic or antiferromagnetic host matrix [1, 2]. Here is investigate low-field (B = 10 G - 1 kG) magnetic susceptibility, c (T), of ceramic LSMFO samples with x = 0.3 and y = 0.15 - 0.25 at temperatures T ~ 5 - 300 K. FM transition in LSMFO is observed at a Curie temperature, TC, decreasing between ~ 210 - 94 K when y is increased from 0.15 to 0.25, which is connected to breaking of the FM double-exchange interaction by doping with Fe. Strong magnetic irreversibility or deviation of the zero-field cooled susceptibility, cZFC (T), from the field-cooled susceptibility, cFC (T), takes place below TC in a field of 10 G, giving evidence for frustration of the magnetic ground state of LSMFO. FM transition is expanded with increasing B, which is connected to sensitivity of the phase separation to the applied magnetic field [2]. The transition is more pronounced at y = 0.15 - 0.20 and broadens considerably at y = 0.25, where the irreversibility is increased and the onset is shifted to temperatures above TC. On the other hand, the magnetic irreversibility is damped with increasing field and disappears completely at y = 0.15 in the field of B = 0.5 kG and at y = 0.20 in the field of 1 kG, whereas at y = 0.25 it persists below ~ 30 K even up to B = 1 kG. The magnetic irreversibility disappears with increasing T. Well above TC, c (T) exhibits a CurieWeiss asymptotic behavior, yielding very large values of the effective Bohr magneton number per magnetic ion (peff 2 ~ 100 - 300), incompatible with those of single magnetic ions, peff 2 = 24, 15 and 35, for Mn 3+, Mn4+ and Fe3+, respectively. This suggests the onset of the phase separation already above 300 K. At y = 0.15 and 0.20 a critical behavior of c-1 (T) ~ (T / TC - 1) g in the region of the FM transition is characterized by influence of two different magnetic systems, a 3D percolative one with g = gp » 1.8 and TC = TC (p) , and a non-percolative 3D Heisenberg spin system, with g = gH » 1.4 and TC = TC (H), where TC (p) < TC (H). At y = 0.25 the percolative contribution to the critical behavior of c (T) is not observed. The ferromagnetism of LSMFO is attributable to percolation over the system of the nanosize FM particles, leading to generation of large and strongly correlated critical FM clusters. On the other hand, the frustration and the irreversible low-field magnetic behavior of LSMFO is governed presumably by a system of smaller weakly-correlated magnetic units, which do not enter the percolative FM clusters. The following parameters, addressed to the onset of the percolation in LSMFO with y = 0.15 - 0.20, are found: the volume fraction of the hole-rich FM phase h » 0.16 - 0.11, the mean radius of FM particles r » 3.4 - 3.8 nm and the mean magnetic moment m » (1.4 - 1.5)×105 mB. Such values are typical of the nanosize FM clusters in the manganite perovskites [2, 3 - 5], supporting the picture of the phase separation effect in LSMFO above.