The Federal Reserve Bank of New York is intensifying its scrutiny of the U.S. units of Europe's biggest banks amid concerns that Europe's debt crisis could spill into the U.S. banking system, say sources familiar with the matter.

The New York Fed is demanding more information from the banks about whether they have reliable access to the funds needed to operate on a day-to-day basis in the United States and, in some cases, pushing the banks to overhaul their U.S. structures, The Wall Street Journal reports.

New York Fed officials "are very concerned" about European banks facing funding difficulties in the United States, a senior executive at a major European bank who has participated in the talks told the Journal.

The New York Fed did not immediately respond to phone calls seeking comment outside regular U.S. business hours.

The officials recently have held meetings with U.S.-based executives from top European banks to discuss their funding positions, the report said.

The New York Fed has also been coordinating with New York's superintendent of financial services, Benjamin Lawsky, to monitor the foreign banks' funding positions.

Regulators are trying to guard against the possibility European banks that encounter trouble could siphon funds out of their U.S. arms, sources told the Journal.

Regulators recently have ramped up pressure on European banks to transform their U.S. businesses into self-financed organizations that are better insulated from problems with their parent companies, the Wall Street Journal cited a senior bank executive as saying.

(Reporting by Soham Chatterjee in Bangalore; Editing by David Cowell)