The European Central Bank (ECB) has asked Greek banks not to increase their holdings of Greek government debt, including Treasury bills, a banking source familiar with the matter told Reuters on Tuesday.

Greece’s international creditors, including the ECB, have set a 15 billion-euro cap on outstanding Greek T-bills which Athens has already hit.

“As supervisors, the ECB and the Bank of Greece are instructing the banks not to increase their exposure to Greek government debt for prudential reasons,” the source said.

Greece, which lives off aid from the European Union, the International Monetary Fund and the ECB, is running out of cash and has asked the ECB to raise the limit. The central bank has refused to do so.

“The ECB and the Bank of Greece have already made clear that further T-bills could not be accepted as collateral,” the source added.