You might go up the Nile river to the city of Luxor, but not beyond. This implies that you actually reach, and are now in, the city of Luxor; but do not go any further.
Similarly, the monitors might observe up to, and including Yongbyon...but not beyond.
The short form of this is then, "...but without access (to observe) beyond (the city of) Yongbyon'.
Yes, you might change it slightly to 'access up to Yongbyon'; but the main issue, what is more important, is that they are unable to observe the rest of the country, what's beyond Yongbyon, where there might be nuclear weapons. How can the rest of the world be sure unless it can 'access all areas'.
Yes - I'll allow the police into my property up to the front door, but I won't allow them beyond the threshold, because inside the house is where I have all the drugs and guns stored. The police aren't so much interested in the area up to my front door - what they are allowed to see - but what's inside, over and beyond the threshold. That's the more important consideration for them!
