* FCC says talks on refinancing debt at advanced stage
* Says potential cash call to be 400-600 mln euros
* Says would back capital hike at Cementos Portland unit
* No decision yet made on buying into Realia cash call (Adds details on capital increase, debt refinancing talks)
By Julien Toyer
MADRID, Nov 12 (Reuters) - Spanish services and construction company FCC said on Thursday it might make another cash call on shareholders to raise up to 600 million euros ($643 million) to repay debt and strengthen its balance sheet.
Mexican billionaire Carlos Slim became FCC's largest shareholder with a 25.6 percent stake at the end of last year after buying more than 50 percent of a $1.3 billion rights issue in a deal with the previous biggest shareholder, Esther Koplowitz, daughter of the company's founder.
But the company has remained mired in debt and has struggled to reshape its business amid still low infrastructure spending in Spain. Its shares were down 3 percent at 6.837 euros by 1022 GMT, extending losses of around 40 percent so far this year.
In a bid to put the group on a firmer footing Slim's Mexican colleague Carlos Jarque was appointed as the new chief executive in August, while talks were opened with creditors to refinance its so-called "tranche B" debt that it would now repay with the proceeds of the new cash call.
Speaking on a conference call with analysts after the release of third-quarter results, the company's head of finance Victor Pastor said the talks were now at an advanced stage and going well.
"We are maintaining a positive and dynamic conversation with creditors. It is a permanent conversation so that we can cancel this 'tranche B' and reach a more efficient capital structure," he said.
"It is an ongoing process, it is a priority objective for us and the quantity (of the capital increase) could range between 400 and 600 million euros."
Pastor also said that FCC would subscribe to any cash call at its subsidiary Cementos Portland in line with its current 78 percent stake.
However, he said no decision had yet been made on whether to buy into a capital increase at property firm Realia, in which FCC holds a 37 percent stake. Realia is controlled by Slim through FCC's stake and a 25 percent stake bought in March by his own property group Inmobiliaria Carso.
The Mexican tycoon is widely expected to take control of the firm after the cash call after he said earlier this week that he would buy into it.
FCC posted on Thursday a net loss of 13.6 million euros in the nine months to September, hit by a weak performance in its domestic market which offset growth in its foreign business.
Core profit (EBITDA) rose 2 percent on the period to 593.4 million euros. ($1 = 0.9299 euros) (Editing by Greg Mahlich)