By Elvira Pollina

MILAN (Reuters) - Italy's top commercial broadcaster Mediaset has raised its stake in German rival ProSiebensat.1 to 15.1% as it presses ahead with plans to create a pan-European TV platform to tackle slow growth and stiffer competition.

In a statement on Monday, Mediaset said its Spanish unit Mediaset Espana had bought a further 5.5% stake in ProSiebenSat.1, worth 170 million euro ($187 million) at Monday's market prices.

The Milan-based company, which is controlled by the family of former Italian Prime Minister Silvio Berlusconi, had bought a 9.6% stake earlier this year.

Mediaset and other traditional European broadcasters are looking for ways to respond to growing competition on their markets from Netflix Inc and online advertising giants like Alphabet Inc's Google and Facebook Inc .

"The transaction confirms Mediaset’s commitment to invest in the future of free TV development in Europe and to unlock growth potential in Germany, Europe's largest media market," the group said.

This recent acquisition makes the Mediaset group ProSieben's largest investor.

A source on Monday said the move by Mediaset could be a way to increase pressure on its German rival to start talks on a possible tie-up.

ProSieben, which so far has shown little inclination to merge the businesses, welcomed Mediaset's move on Monday as affirmation of its strategy, saying it remained fully focused on building more local content and expanding its digital reach.

Mediaset is trying to push ahead with a corporate overhaul and merge its Italian and Spanish business into a Dutch holding company, dubbed MediaforEurope, to pursue pan-European tie-ups, including a potential merger with ProSiebenSat.1.

Vivendi, which is Mediaset's No. 2 shareholder, is challenging the plan in the courts claiming it includes a governance structure that would help Fininvest, the Berlusconi family's holding company, tighten its grip over the broadcaster.

But the two groups have recently engaged in talks about ways to overcome a dispute over the governance of the new entity.

(Reporting by Elvira Pollina, additional reporting by Alexander Hubner, editing by Stephen Jewkes and Lisa Shumaker)