Prime Minister Gordon Brown has condemned Iceland's handling of the collapse of its banks and its failure to guarantee British savers' deposits.
Mr Brown said it was "effectively illegal" and "completely unacceptable".
Mr Haarde said Iceland had not decided whether to seek help from the International Monetary Fund to weather the crisis.
The country's Financial Supervisory Authority said it took over Kaupthing to safeguard its domestic banking system.
All domestic deposits at the bank were fully guaranteed, it added.
On Wednesday, the UK Treasury arranged for ING Direct to take over the £2.5bn of deposits of 160,000 UK customers of Kaupthing's online arm, Kaupthing Edge.
The Swedish central bank had already agreed to provide a loan to the bank's Swedish arm.
Iceland's government has now seized control of all three of the nation's major banks. Landsbanki and Glitnir were taken over earlier this week.
The country of just 300,000 people has struggled to cope with the global financial crisis.
"The action taken... was a necessary first step in achieving the objectives of the Icelandic government and parliament to ensure the continued orderly operation of domestic banking and the safety of domestic deposits," Iceland's Financial Supervisory Authority said.
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