WARSAW (Reuters) - Poland will invest 10 billion zlotys ($2.55 billion) in a programme to secure its eastern border, the prime minister said on Saturday, in a bid to bolster its defences against what it says is a rising threat from Russia and Belarus.

Poland's border with Belarus has been a flashpoint since migrants started flocking there in 2021, after Minsk, a close Russian ally, opened travel agencies in the Middle East offering a new unofficial route into Europe - a move the European Union said was designed to create a crisis.

Since Russia's 2022 invasion of Ukraine, relations have become even tenser, with Warsaw ramping up defence spending and accusing Minsk and Moscow of attempts to destabilise Poland.

"We have decided to invest 10 billion zlotys in our security, and above all in a secure eastern border," Prime Minister Donald Tusk told a news conference.

"We are starting a major project to build a secure border, including a system of fortifications as well as landscaping and environmental decisions that will make this border impossible to pass for a potential enemy."

Tusk did not give further details of what kinds of fortifications would be built.

The previous government built a fence on the Polish-Belarusian border that was over 180 km long and 5.5 metres high, to protect against illegal migration. It is complemented by a system of cameras and sensors monitoring the frontier.

Tusk had announced plans to strengthen the eastern border earlier in May, but without giving details.

Tusk also said that he would speak with the European Investment Bank on Monday about financing of 500 million zlotys for the satellite component of the European Sky Shield Initiative.

The European Sky Shield Initiative is a common air defence scheme set up by Germany in 2022 to boost European air defence. Tusk has compared it to Israel's Iron Dome system.

($1 = 3.9157 zlotys)

(Reporting by Alan Charlish; Editing by Andrew Cawthorne)