MILAN (Reuters) - Shares in Telecom Italia SpA (>> Telecom Italia SpA) fell almost 6 percent on Friday, on disappointment at a lack of detail and no timeframe in a spinoff plan put forward by the company late the previous session.

Telecom Italia agreed on Thursday to put some of its network assets - valued at between 13 billion euros $17 billion (11 billion pounds) and 15 billion - into a separate company, with a view to selling a stake in that new entity to state-backed fund Cassa Depositi e Prestiti (CDP).

But it offered little detail on the project, which requires regulatory changes and must be discussed with local watchdog AGCOM.

A source familiar with the matter said Telecom Italia had informed AGCOM about its plan, but more information was needed before the regulator could take a decision, a process that could take a couple of months.

"We expect all the operators to be involved in this process and therefore full visibility on the implications for Telecom Italia could take some time," analysts at Espirito Santo said.

Ownership of the network implies Telecom Italia must abide by stricter rules than its rivals Vodafone (>> Vodafone Group plc), Fastweb (>> Swisscom AG) and Vimpelcom's (>> VimpelCom Ltd (ADR)) Wind. The Italian group wants these restrictions to be removed before the spin off, people familiar with the issue have said.

Shares in Telecom Italia were down 5.7 percent at 0.60 euros by 3.31 p.m. British time, backtracking after rising 14 percent in the past three months.

One source with direct knowledge of the matter said the spinoff could take up to 18 months to complete, although a deal with CDP could take less. CDP chairman Franco Bassanini on Friday said the spinoff ought to be carried out rapidly.

The idea of putting Telecom Italia's fixed-line copper and fibre network into a separate company follows political concerns over a proposed tie-up with Hutchison Whampoa (>> Hutchison Whampoa Limited).

Telecom Italia had said it would discuss a tie-up with the Hong Kong-based conglomerate's Italian mobile phone unit, 3 Italia, at a board meeting due to take place on June 5.

(Editing by Lisa Jucca and David Holmes)

By Danilo Masoni