Private owners of Greek bonds will accept a 50 percent writedown on their investment, enabling both a 100 billion euro cut in the country's sovereign debts and allowing a new programme of aid of 100 billion euros, German Chancellor Angela Merkel said on Thursday.

"Our goal is that the debt of Greece by 2020 is 120 percent (of GDP)," Merkel told journalists after a meeting of eurozone leaders.

"A nominal haircut of 50 percent has been agreed. On the basis of this, we will have a new programme for Greece with a value of 100 billion euros."

She said the public sector would make a further 30 billion euro contribution towards private sector participation.

Merkel also said that the country would be subject to closer supervision in future.

"There will be a reinforced monitoring regime in connection with the fulfilment of the Greek obligations," she said.

"That will be anchored in a memorandum of understanding. There will be a permanent presence there. It will be possible to monitor the measures taken by Greece. I think that this is better than when every three months a 'troika' travels there and back, a permanent system of supervision."