NSF the US federal agency whose mission includes support for all fields of fundamental science and engineering, except for medical sciences. NSF operates from the "bottom up", which means that there are no pre-conditioned calls for proposals. NSF rarely provides support to foreign organizations. However, proposals for cooperative projects involving U.S. and foreign organizations will be considered.
There is an office of the NSF specifically devoted to encouraging international collaboration. You will find specific funding opportunities earmarked for collaboration with a particular country.
Web: https://www.nsf.gov/funding/index.jsp