Banks are usually reluctant to use the ECB's instant access facility - also known as its emergency window - because it charges 0.75 percentage points more interest than normal ECB funding.

Thursday's uptake of 3.9 billion euros added to a steady rise in use of the facility in recent days, after the central bank cut four Greek banks off from its mainstream funding last week.

The ECB does not disclose which banks use the emergency funding.

The central bank's action against the Greek banks pushed them to use Emergency Liquidity Assistance (ELA), which is more costly than the ECB's mainstream funding and for which Greece rather than the euro zone as a whole is ultimately responsible.

The ECB's overnight window is a constantly available option for banks, while ELA is a temporary facility reserved for times of stress that can only be tapped once ECB's policymakers give their approval.

"If (funding at the overnight window) ...rose above 5 billion, I'd start to be concerned. But I would need to see more of a pattern," said one London-based money market trader who requested anonymity.

"This could be a failure of one payment late in the day. If it was a liquidity issue, it was probably from one of the southern banks. But it might just as well have been between two very well known large banks."

The ECB also revealed the full impact of its move on the four Greek banks, publishing a breakdown of the reduced amounts being lent out following the cutback.

In total, the reductions added up to 31 billion euros with 17.4 billion cut from the first of its recent twin 3-year loans and the rest from one and three-month funding.

(Reporting by Marc Jones and Eva Kuehnen; Editing by John Stonestreet)

• A Permanent Economic Emergency