European Union will transfer to Poland this year a first 5 billion euros ($5.5 bln) of financial aid that was until now mostly frozen over democratic backsliding, the bloc's chief executive and the country's new prime minister said on Friday.

Poland's new Prime Minister Donald Tusk vowed to restore the rule of law in the EU's largest eastern state after ousting from power the nationalist Law and Justice (PiS) that fought bitter battles with the EU for nearly a decade over democratic rights.

"I welcome your commitment to put the rule of law at the top of your government's agenda," European Commission President Ursula von der Leyen said after a meeting with Tusk in EU hub Brussels.

In announcing the first transfer - which comes under an EU programme to help energy transition away from fossil fuels and is free of the bloc's usual rule of law conditions on handouts - von der Leyen said more money would come as Poland restores an independent judiciary.

"We will need to make up for lost time," she said.

She welcomed Warsaw's aim to join the bloc's European Public Prosecutor's Office (EPPO), something PiS refused to do.

In welcoming what he quipped was a Christmas gift from the EU executive, Tusk said Poland would rebuild democratic institutions and play a more active role in Europe, another change seen in Warsaw after years of a deepening PiS isolation.

"To be continued," he wrote on social media. ($1 = 0.9132 euros) (Reporting by Gabriela Baczynska; editing by Philip Blenkinsop)