The following graph shows the major components of the federal debt:
The red line is the the "gross federal debt" which is currently about $8 trillion. The space between the red and green lines represents that part of the debt that is owed to the Social Security trust fund and the space between the green and blue lines represents that part of the debt that is owed to other trust funds and government agencies. Finally, the blue line is the "debt held by the public" and the gray line is the part of that publicly held debt that is foreign-held. The actual numbers and sources can be seen at
http://home.att.net/~rdavis2/debt06.html .
As can be seen in the table at the above URL, about 44 percent of the public debt was foreign-held at the end of 2004. During 2004, however, the total public debt increased $382.1 billion and the foreign-held public debt increased $430.2 billion. Hence, in effect, the total increase was borrowed from foreign sources and an additional $48.1 billion of existing debt passed from domestic to foreign hands. Over the past three years, the increase in the foreign-held public debt has been 90.2 percent of the increase in the total public debt.
You can find a little more information on the foreign held debt at
http://www.treas.gov/tic/mfh.txt . According to it, the foreign-held debt at the end of last October was about $2102 billion. Of this, $1227.9 billion (about 58%) was "Foreign official". I think this refers chiefly to central banks. In any case, you can also find a breakout by nations at that URL.