By Silvia Ognibene and Steve Slater
SIENA/LONDON, Italy (Reuters) - Italian prosecutors said they have ordered the seizure of 1.8 billion euros ($2.4 billion) of assets from Japanese bank Nomura <8604.T> as part of an investigation into a suspected fraud involving troubled lender Monte dei Paschi di Siena
Prosecutors in the city of Siena also said they were investigating Nomura's former top executive in Europe, Sadeq Sayeed, and Raffaele Ricci, managing director in fixed income sales for the EMEA region at the Japanese bank.
The two men are being investigated over allegations of aggravated fraud and usury (overpriced loans), obstructing the work of regulators and making false statements to the market in dealings with Monte dei Paschi di Siena.
Nomura said that none of its assets had been seized in connection with the Monte dei Paschi probe and rejected any suggestion of wrongdoing.
"We will take all appropriate steps to protect our position and will vigorously contest any suggestions of wrongdoing in this matter," the bank said in a statement on Tuesday.
Sayeed, who left Nomura in March 2010, also denied the allegations. "The first I heard of it was in the reports earlier today," he told Reuters. "I have not been approached by anybody. I am trying to analyze it myself."
Ricci did not immediately return calls for comment.
The prosecutors said in a statement that the seizure order concerned 88 million euros of hidden commissions that they say Nomura received and 1.7 billion euros of funds deposited with Nomura by Monte dei Paschi by way of collateral for the so-called Alexandria trade, which is at the center of an investigation into risky derivative deals.
The trade involved the purchase by Monte dei Paschi of Italian government bonds for 3 billion euros, which the bank financed through a long-term repurchase agreement with Nomura.
Prosecutors and the bank's current management say the trade helped Monte dei Paschi to conceal losses by spreading them over 30 years.
Smaller sums were being seized on Tuesday from former Monte dei Paschi chairman Giuseppe Mussari, ex-managing director Antonio Vigni and the former head of the finance department, Gian Luca Baldassarri - all of whom are under investigation over the Alexandria deal and other trades.
Mussari and Vigni declined to comment. Baldassarri was arrested on February 14.
The 2009 transaction between Nomura and the Tuscan bank is one of a series of loss-making "structured finance" deals carried out under the Tuscan lender's previous management to conceal losses, according to prosecutors.
The bank, which in February received a state bailout of 4 billion euros that was also meant to plug a capital shortfall exacerbated by the derivatives deals, booked a pretax loss of 730 million euros in 2012 linked to those trades.
The bank's new management has said it only discovered the true nature of the Alexandria deal in October 2012 after finding a document hidden in a safe.
A judicial source said the seizure order against Banca Nomura International Plc, a unit of the Japanese bank, had effectively frozen the Alexandria transaction.
"The seizure order interrupts the validity of the contracts so they cease to have effect," the source said. "The contracts have been frozen, so receipts and payments are suspended."
Last month Monte dei Paschi, the world's oldest bank, filed a lawsuit in Florence seeking at least 700 million euros from Nomura, Mussari and Vigni over the Alexandria trade. Nomura has been trying to establish British jurisdiction for the dispute.
The Tuscan lender is also seeking at least 500 million euros in compensation from Mussari, Vigni and Deutsche Bank AG
Deutsche Bank has said that its 2008 trade with Monte dei Paschi was subject to rigorous internal approval processes.
Monte dei Paschi has been rocked by the scandal over the opaque derivatives and by another investigation into its costly 2007 acquisition of rival Banca Antonveneta. ($1 = 0.7616 euros)
(Writing by James Mackenzie and Silvia Aloisi; Editing by David Holmes and David Goodman)