Context : The nature of embedded accretion disks around forming high-mass stars is one of the missing puzzle pieces for a general understanding of the formation of the most massive and luminous stars .

Aims : We want to dissect the small-scale structure of the dust continuum and kinematic gas emission toward two of the most prominent high-mass disk candidates .

Methods : Using the Plateau de Bure Interferometer at \sim 1.36 mm wavelengths in its most extended configuration we probe the dust and gas emission at \sim 0.3 ^ { \prime \prime } , corresponding to linear resolution elements of \sim 800 AU .

Results : Even at that high spatial resolution NGC7538IRS1 remains a single compact and massive gas core with extraordinarily high column densities , corresponding to visual extinctions on the order of 10 ^ { 5 } mag , and average densities within the central 2000 AU of \sim 2.1 \times 10 ^ { 9 } cm ^ { -3 } that have not been measured before .
We identify a velocity gradient across in northeast-southwest direction that is consistent with the mid-infrared emission , but we do not find a gradient that corresponds to the proposed CH _ { 3 } OH maser disk .
The spectral line data toward NGC7538IRS1 reveal strong blue- and red-shifted absorption toward the mm continuum peak position .
While the blue-shifted absorption is consistent with an outflow along the line of sight , the red-shifted absorption allows us to estimate high infall rates on the order of 10 ^ { -2 } M _ { \odot } yr ^ { -1 } .
Although we can not prove that the gas will be accreted in the end , the data are consistent with ongoing star formation activity in a scaled-up low-mass star formation scenario .
Compared to that , NGC7538S fragments in a hierarchical fashion into several sub-sources .
While the kinematics of the main mm peak are dominated by the accompanying jet , we find rotational signatures from a secondary peak .
Furthermore , strong spectral line differences exist between the sub-sources which is indicative of different evolutionary stages within the same large-scale gas clump .

Conclusions : NGC7538IRS1 is one of the most extreme high-mass disk candidates known today .
The large concentration of mass into a small area combined with the high infall rates are unusual and likely allow continued accretion .
While the absorption is interesting for the infall studies , higher-excited lines that do not suffer from the absorption are needed to better study the disk kinematics .
In contrast to that , NGC7538S appears as a more typical high-mass star formation region that fragments into several sources .
Many of them will form low- to intermediate-mass stars .
The strongest mm continuum peak is likely capable to form a high-mass star , however , likely of lower mass than NGC7538IRS1 .