In the case of individual self-defence, the exercise of this right is subject to the State concerned having been the victim of an armed attack. Reliance on collective self-defence of course does not remove the need for this. There appears now to be general agreement on the nature of the acts which can be treated as constituting armed attacks. In particular, it may be considered to be agreed that an armed attack must be understood as including not merely action by regular armed forces across an international border, but also ‘the sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to’ (inter alia) an actual armed attack conducted by regular forces, ‘or its substantial involvement therein’.
it may be considered to be X
is equivalent to
[for the purposes of the current discussion] let us agree that it is in fact X
So in the cited context, all the writer is really saying is let us agree to agree [that an armed attack must be blah blah]. Which is just a long-winded way of saying An armed attack is blah blah. Don't copy this style.
It is neither agreed upon nor denied. It is intentionally left ambiguous so as to decide on a case-by-case basis.
It is only agreed to consider acts of armed bands, groups, irregulars or mercenaries acting on behalf of a State as an actual armed attack conducted by regular forces. However it may not directly be agreed that all acts of such entities might constitute an actual armed attack from the State. Because the extent and extremity of an attack varies remarkably.
"It may be considered to be agreed" is the way of saying we will consider such external acts as an actual attack by a State, but we need to investigate it further before we decide.