Sure it can. For the US FDA, it is considered offshore supportive data (if a good safety profile and/or a pharmacodynamic effect is observed) irrespective if it is a first in man trial or P2 or P3 and must be conducted under the auspices of the foreign regulatory authorities. Of course a US IND would have to be opened for any US human clinical studies. In most cases and at some point the FDA would require US trials as part of the registration package. "Conclusive" is not the right word here. Small studies can be viewed as trending, larger proof of concept trials P2 as well as P3 pivotal trials use terms such as "Safe and Effective" with statistically meaningful endpoints. These are the regulatory descriptors used by the US FDA.
News 
Market Data 
Discover 
