Donald Tusk has said Poland's future inside the EU is 'uncertain' amid an escalating row with the country's ruling conservative party.

The European Council President, who was the country's Prime Minister until 2014, said he 'understands' the concerns of Poles who are fearful about the direction their country is taking.

President Andrzej Duda and his Law and Justice party have been accused of undermining democracy as they exert growing control over state media, and attempt to seize power from the judiciary.

Donald Tusk has said Poland's future inside the European Union is 'uncertain' amid a deepening row with Jaroslaw Kaczynski, leader of the country's ruling conservative party

Tusk spoke after testifying at a trail over a 2010 plane crash that killed President Lech Kaczynski - Jaroslaw's twin brother - and 95 other senior government figures

Tusk spoke in Warsaw after testifying in a case related to the 2010 plane crash that killed President Lech Kaczynski and 95 other senior officials.

Some see the trial as a politically-motivated attempt to discredit Tusk driven by Jaroslaw Kaczynski, the leader of the Law and Justice party and the twin brother of the dead former President.

Tusk said: 'There is a question mark over Poland's European future today.

'I do understand emotions of Poles who are concerned about courts, or Poland's future in the EU.'

Kaczynski holds no government position but is widely seen as the man who directs all major government decisions.

Ahead of Thursday's questioning in Warsaw, Kaczynski warned that Tusk 'should be afraid'.

Kaczynski has also suggested that Germany should pay 'huge' reparations to Poland for crimes committed by the Nazis during the Second World War.

'We are talking about huge sums and also the fact that the Germans have for years rejected their responsibility for World War II,' he said.

While tensions between Tusk and Kaczynski have been simmering for years, the latest crisis was caused when his party attempted to reform the Supreme Court.

Kaczynski's Law and Justice party, which won power from Tusk's own party in 2015, has attempted to reform the country's media and Supreme Court in a way that would hand massive powers to the government in a move that many consider undemocratic

The government recently attempted to pass a bill that would allow ministers to force Supreme Court judges into retirement before picking their replacements.

It passed both houses of the national assembly and was only stopped after widespread protests pressured President Dudas into vetoing it, in a major snub to Kaczynski.

Tusk's centrist Civic Platform party was in power for eight years until the Law and Justice party won the 2015 elections and swiftly moved to introduce reforms.

Although some Poles view their judiciary as corrupt and dominated by communist-era ways of thinking, others see the PiS-driven reform efforts as a power grab inimical to democracy.

'There are plenty of issues where the Polish government's actions seem very controversial from the point of view of the whole EU. Including Budapest, sometimes,' Tusk said.

Hungary, normally Poland's ally, voted for Tusk's re-election to the top EU post in March, disappointing the PiS.

Tusk also criticised Prime Minister Beata Szydlo's government over the logging in the primeval Bialowieza Forest.

The European Court of Justice ordered an immediate halt to the logging last week, saying that Warsaw's attitude in the case hinted at 'a prelude to an announcement that Poland does not need the European Union and the European Union does not need Poland.'