ROME (Reuters) - The chief executive of Italian infrastructure group Atlantia (>> Atlantia) does not rule out sweetening the terms of an offer for Spanish rival Abertis (>> Abertis Infraestructuras), which could become the target of a counterbid.

"It is too early to say" whether Atlantia could raise the price of its offer, CEO Giovanni Castellucci said on the sidelines of a shareholder meeting in Rome.

Atlantia in May offered 16.3 billion euros (14.36 billion pounds) in a cash-and-equity bid for motorway operator Abertis.

The Italian group submitted its proposal to the Spanish regulator in June and is still waiting for a response both from the watchdog and from Abertis' main shareholders.

Meanwhile, Spanish builder ACS (>> ACS Actividades de Construccion y Servicios) has said it may launch a counterbid for Abertis together with other investors.

Asked about the possibility of an all-cash offer, Castellucci said Atlantia was not planning any changes to the structure of the bid but added nothing could be ruled out.

"We think we have broad enough shoulders," he said when asked about a possible bidding war with ACS.

Atlantia's shareholders approved on Wednesday a capital increase of up to 3.79 billion euros through the issue of up to 16.3 million special shares that will be handed to Abertis' shareholders if the planned takeover goes ahead.

The shareholders also voted in favour of an incentive plan for a group of managers, from both Atlantia and Abertis, who would be involved in the tie-up between the two companies, in a further move to woo the executives of the Spanish group.

To win the support of Abertis' managers and investors, Atlantia has also promised to keep the motorway operator listed in Madrid and maintain its headquarters in Spain.

With a 22.3 percent stake, the top investor in Abertis is Criteria Caixa, the financial arm of a politically connected and powerful banking foundation.

($1 = 0.8453 euros)

(Writing by Francesca Landini; Editing by Mark Potter)

By Giuseppe Fonte