* Foreign groups interested in Ansaldo STS, AnsaldoBreda
* Unions oppose ceding control to foreign groups
* Finmeccanica shares rally 4.5 pct
MILAN, Sept 23 (Reuters) - Finmeccanica told unions on Monday it is in advanced talks to sell its prized gas turbine unit AnsaldoEnergia to Korea-based Doosan Heavy Industries, a move that would help Italy's defence and aerospace group cut its large debt pile.
At a meeting with unions, Chief Executive Alessandro Pansa also said the state-backed group was in talks with foreign manufacturing companies for the possible sale of unprofitable train maker AnsaldoBreda and rail technology group AnsaldoSTS, two other units that are part of Finmeccanica's disposal plan.
Shares in Finmeccanica rose as much as 4.5 percent after the update on a sale disposal plan worth around 1 billion euros that has been in the works for more than a year.
Rating agencies Fitch, Standard and Poor's and Moody's have all cut Finmeccanica's debt to 'junk' after political wrangling and a corruption investigation into an Indian helicopter tender slowed down the disposal plan.
In a statement, unions said they opposed the sale of the units to foreign groups unless prospective buyers gave clear reassurances that jobs and key technological know-how would stay in Italy, now in its second year of recession.
"We believe the Italian government should immediately get involved. We ask the government to clarify once and for all its position with regards to the sale process," the unions said.
Unions would like Italy's state-holding Cassa Depositi e Prestiti or its Italian Strategic Fund investment arm to buy a core stake in the three Finmeccanica's units to avoid ceding control to foreign players.
A union source said one of the options on the table was to sell a portion of Ansaldo STS together with AnsaldoBreda to make the latter more appealing to a prospective buyer.
Finmeccanica had 3.4 billion euros in net debt at end-2012.