Eh, drone strikes by a sovereign military are by definition not terrorist - they are acts of war.
I really need to word this better, but the acts must surely must be against a state or its people as covered by the state-sanctioned umbrella. If the (intended) target was already one (theoretically) outlawed[1] by the state, probably an enemy of the Taliban already subject to legal sanctions that go
at least up to judicious execution without prejudice by the state's own forces, then this is not war against the state. The state may even appreciate (at least secretly) the intervention, saving themselves at least the temporary worry of having had a non-state actor within their state and threatening the
state of their state
For the (if there was one) non-Taliban intended killer, that is. The collateral damage to persons and property that were still state-covered is a different issue. It would be an issue under international law similar in style (but maybe not magnitude) to any other cross-border recklessness such as (non-belligerant) poisoning of waters that flow between the territories, or burning a forested area that recklessly crosses the borders. Or allowing a disfunctional plane to take off that then crashes onto a neighbouring (or further) territory. Getting the aim of a live-fire military exercise/missile test wrong, too near a border, might be closer still in equivalence (assuming no intention to 'accidentally' bombard the end-zone, something that might be the case for somewhere like Best Korea).
But IANAInternationalL. The subtleties are also tied quite a bit to the respective policies and opinions between the sides involved (all three/four of them) and have surely been tried and tested with the likes of Mossad hit-squads on foreign soil (if not the kinds of people not entirely unlinked to a state of their own, part of the typical target-group for Mossad hit-squads upon mutually foreign soils, etc). It could involve Strong Words™, or it could be taken up to the level of grounds for declared warfare.
In this case (based upon my current range of assumptions, as already outlined) the US is culpable for any number of things which certainly
could lead to severe repurcussions from those countries which have enough clout to censure or even treat this as a permissive precedent of some kind, but the state of war between the US and the current Afghani state is unaltered (it may actually be consistently as 'on' as it managed to survive as a true war, even as the warred-against Talib were no longer a State to properly be at war with, but that's a question for someone with a Statesman's paygrade to think about, and a bunch of them from across various boundaries to try to develop an official concensus about) and what is left, from a purely Afghani point of view, is an International Incident where no serious declaration of war would be imagined by either party (the wronged or the wronger), although blowhards might try to capitalise upon the issue with rhetoric upon the issue.
IMO. Don't come to me to validate the legality of what I'm suggesting. Maybe the UN Secretary General (amongst others) has the contact details of those that could, but he's not likely to be browsing these forums and throwing around such opinions.
Much regards,
António
-- (written on my iPad)
[1] In the classical sense of one not held to be protected by the standard measures of the law, rather than the somewhat similar and co-existing quality of being an on-the-run criminal for whom every act is effectively a continuing illegality requiring legal punishment at the earliest opportunity. Basically, Robin Hood was an outlaw because he had no recourse to the courts (to whatever extent he might have had otherwise) should he ever have a grievance to settle, not because of the acts of rampant longbowing he continued to inflict upon anyone else once declared outwith the law. Although the desire to continue to run amok certainly sustains the status.