Ukrainian President Volodymyr Zelensky on Monday denounced a weeks-long border blockade by Polish truckers and farmers which his government has said threatens the country's security.

Polish farmers have been blocking Ukrainian cargo to protest against what they say is unfair competition.

The row has strained relations between the neighbors, even though Poland has been one of Ukraine's staunchest supporters since the start of the war in February 2022.

"Things that are happening on our western border, the border with Poland, cannot be considered normal or ordinary," Zelensky said, adding the situation "demonstrates the erosion of solidarity on a daily basis."

Road traffic into EU member Poland has been an export lifeline for Ukrainian companies after the war in Ukraine held up major trading routes through the Black Sea.

But the increased shipments have angered Polish logistics companies and farmers, who say their Ukrainian rivals are undercutting them.

"In reality, the situation is not about grain, but rather about politics," Zelensky said, arguing that only five percent of agricultural exports went through the Polish border.

Six border crossings were being blocked on the Polish side, Infrastructure Minister Oleksandr Kubrakov said on Monday.

"Blocking the border is a direct threat to the security of a defending country," Kubrakov said on Facebook.

"They are blocking everything. There are trucks with fuel. There were cases a few days ago when protestors didn't let several trucks with weapons pass," he also told state TV.

A Polish official said there was a queue of around 600 trucks waiting to leave Poland for Ukraine at the Dorohusk border crossing.

The situation is expected to intensify on Tuesday when Polish farmers are calling for a halt to all border traffic with Ukraine.

Read more: Ukraine's grain complaint with WTO against Poland still pending