The size of the US commercial paper market grew $4.9 billion to $968 billion in the week ended Jan. 18 on a seasonally adjusted basis. This was the highest level in four weeks.

Not including seasonal factors, this short-term corporate debt market grew $4.6 billion to $1.014 trillion in the latest week.

Foreign banks have ratcheted up their borrowing in early 2012 after money market funds and other investors slashed their holdings of commercial paper from euro zone banks in the second half of 2011 due to that region's sovereign debt crisis.

Foreign bank commercial paper outstanding on non-seasonally adjusted basis raised $2.8 billion to $133.3 billion in the latest week, according to latest Fed data. US banks with foreign parents issued $6.1 billion in commercial paper, bringing their total to $152.1 billion in the latest week.

The non-seasonally adjusted sum of the highest-rated commercial paper that money market funds could buy rose by $4.9 billion to $738.6 billion. Money market funds buy roughly half of all commercial paper issued.