STOCKHOLM (Bloomberg) — The Swedish Financial Supervisory Authority sent a letter to Borse Dubai asking to clarify "issues" related to the purchase of a 27.4 percent stake in the Swedish exchange operator OMX AB.

The letter sent to Borse Dubai was "to clarify certain issues by noon this Tuesday," Helena Oestman, director of communications at the Swedish regulator, said in a telephone interview.

She declined to give details about the "issues" nor comment on whether Borse Dubai had acknowledged receipt of the letter sent yesterday.

The purchase by the owner of the emirate's two stock exchanges of the OMX stake has come under regulatory scrutiny, the Financial Times said.

Borse Dubai must submit a formal offer document within four weeks and meet other take-over conditions if it's making a bid, the FT said, citing Annika von Haartman, head of prospectuses and listings at Sweden's Financial Supervisory Authority.

Borse Dubai separately confirmed that it had received a letter from the FSA containing "routine" queries and said it had done nothing against the law, the FT reported.

Regulators said that if they determine the move is a bid and Borse Dubai disagrees, they could fine the Dubai exchange as much as 100 million Swedish krona ($14.8 million) and ban it from bidding for OMX, the FT added.

"We don't have any comment at the moment," Heidi Wendt, vice president and internal communications head at OMX, told Bloomberg News today in response to the letter sent to Borse Dubai.

Borse Dubai is challenging Nasdaq Stock Market Inc.'s bid for OMX after winning control of the stake in the Swedish exchange operator.

Borse Dubai acquired the shares and options at a price 11 percent higher than Nasdaq's 26 billion-krona takeover offer.

Nasdaq Chief Executive Officer Robert Greifeld said the New York-based exchange has the financial resources to alter its cash-and-stock bid to complete the transaction, which will enable it to expand into Europe.

Borse Dubai owns 4.9 percent of Stockholm-based OMX after buying shares for 230 krona each and has options for 22.5 percent, the exchange said last Thursday.

The exchange would require regulatory approval to exercise the options.

The purchase price values OMX at 27.7 billion krona.

Dubai's investment may open a take-over battle for OMX and threaten Greifeld's 17-month-old strategy of extending Nasdaq's reach into Europe through acquisitions.

Nasdaq has already failed to buy London Stock Exchange Plc while rival NYSE Group Inc. created the first transatlantic stock market with the $14 billion acquisition of Paris-based Euronext in April.