* Vinci to play a role if ADP opportunity arises - CEO
* Unclear yet what French state will decide on ADP - CEO
* Vinci has 8 pct of ADP
* Shares fall but Vinci still outperforming this year (Recasts with CEO comments on ADP)
PARIS, July 28 (Reuters) - Vinci, Europe's biggest construction and concessions company, said it could invest more in Aeroports de Paris (ADP) if the French state decides to privatize the airports operator or sell part of its stake.
Vinci also reiterated its forecasts for higher annual earnings and revenue after strong momentum at its airports and motorway concessions businesses coupled with a recovery in French construction lifted its first-half results.
Vinci has expanded into faster growing and more profitable concessions such as airports, motorways and energy engineering, to counter a recent downturn in the French construction sector.
"If there is an opportunity on ADP, we will try to play a role," Vinci Chairman and Chief Executive Xavier Huillard told reporters. "We are interested in airports. We already have an 8 percent stake in ADP, which is a nice asset."
Huillard added though that it was unclear at this stage what the French state might decide to do with ADP.
The government plans to reduce its corporate holdings in the coming months to finance a fund for investment in innovation.
While it has not said which stakes will be sold, sources said in June the state would probably start its privatization program with ADP, in which it holds 50.6 percent.
Vinci, which secured a stake in Lyon airport in France along with public sector lender Caisse des Depots (CDC) last year, operates 35 airports worldwide.
Earlier this month, it bought a 51 percent stake in Portugal's airport retailer Lojas Francas Portugal and was named preferred bidder for the Kobe airport in Japan.
In France, Vinci's building businesses benefited from a buoyant property sector, while Vinci also won contracts under the "Grand Paris Express" expansion plan for the capital.
The French construction sector has been in recession over the last three years but the Vinci CEO said it was now "on a recovery trend."
Vinci shares were down 2.5 percent in mid-session trading, tracking a fall on European stock markets, although the stock remains about 16 percent higher so far in 2017, outperforming a 5 percent rise in the benchmark CAC-40 index. ($1 = 0.8558 euros) (Reporting by Dominique Vidalon; Editing by Andrew Callus and David Clarke)