(Adds comments from Argentine finance secretary paragraphs 6-8)

By Richard Lough and Stefano Bernabei

BUENOS AIRES/ROME, Feb 2 (Reuters) - Argentina has reached a deal to pay $1.35 billion in cash to a group of Italian creditors who hold unpaid sovereign debt stemming from the South American country's record default in 2002, the investors said on Tuesday.

The deal, which is subject to approval in Argentina's Congress, represents a payment of 150 percent on the $900 million principal value of the defaulted bonds.

"That means the entire nominal value plus 50 percent interest," Nicola Stock, president of Task Force Argentina which grouped together some 50,000 bondholders, told Reuters.

Even though newly elected President Mauricio Macri does not hold a majority in Congress, Stock said he expected lawmakers to greenlight the deal swiftly once it returns from recess in early March.

Finance Minister Alfonso Prat-Gay earlier said the Italian investors' holdings accounted for 30 percent of all the debt that is subject to legal claims in a U.S. federal court and 15 percent of the defaulted debt that was not restructured in 2005 and 2010. Argentina defaulted on $100 billion of debt in 2002.

In New York, Argentine Finance Secretary Luis Caputo said mediated talks with New York hedge funds leading litigation in the U.S. courts for full payment were "making progress."

He said Argentina could make an offer to the funds on Wednesday or Thursday, though there are no signs a deal is close.

Asked how far apart the two sides were, Caputo said: "I wish I knew."

While the agreement with Italian creditors is a boost for Macri, it may not strengthen his government's negotiating hand.

"Argentina still has no leverage in that they want the deal more than the bondholders," said Siobhan Morden, head of Latin America fixed income strategy at Nomura.

Argentina needs a deal to emerge from default and tap global credit markets at more affordable rates.

Last month, Prat-Gay said the previous government's failure to reach a deal with holdout creditors had cost the economy, with creditor claims in New York rising to $9.9 billion from $2.943 billion initially.

Prat-Gay stressed on Tuesday that Argentina was committed to finding a quick and fair settlement with the U.S. investors.

The preliminary accord with some 50,000 Italian bondholders underlines the divergent views among the hedge funds and other "me-too" claimants who have joined the litigation about what an acceptable agreement looks like.

"The difficulty that we have is that some bondholders want to receive a level of interest that is unacceptable under any legal criteria," Prat-Gay said.

(Additional reporting by Daniel Bases in New York; Writing by Richard Lough; Editing by Chizu Nomiyama, W Simon, Lisa Shumkaer and Bernard Orr)