* Foreign Treasuries holdings book biggest jump since 2007
* Foreign cenbank agencies holdings slip below $300 bln
NEW YORK, March 27 (Reuters) - Foreign central bank holdings of U.S. Treasuries at the Federal Reserve rose for a second straight week following a record drop three weeks ago, according to Fed data released on Thursday.
Overseas central banks increased their U.S. government debt holdings by $56.3 billion to $2.94 trillion this week, building on the prior week's $32.2 billion rise.
The jump in their ownership of Treasuries was the biggest weekly increase since the Fed data were available going back to mid-2007.
Three weeks earlier, foreign central banks' Treasury holdings fell an eye-popping $104.5 billion, more than three times the previous record decline.
It remained unclear what caused that enormous drop in foreign holdings.
Suspicions centered on Russia cutting its dollar reserves ahead of possible sanctions from the West due to its role in the Ukraine crisis, but it is possible other major holders of U.S. debt, such as China and Japan, could have been big sellers as well, perhaps to support their own currencies.
Some analysts reckoned Moscow simply shifted its Treasuries holdings outside the United States rather than dumping them on the open market.
U.S. President Barack Obama has put in place financial sanctions on members of Russian President Vladimir Putin's inner circle and held out the prospect of sectoral sanctions, including in the defense sector, if Russia makes further military moves against Ukraine.
On Thursday, the U.S. State Department imposed a ban on the issuance of licenses for the export of defense items and defense services to Russia in response to Russia's annexation of Crimea.
While foreign banks' holdings of Treasuries with the Fed rose in the latest week, their ownership of agency debt with the central bank slipped by $1.57 billion to $299.0 billion. (Reporting by Richard Leong; Editing by Meredith Mazzilli)