Ukraine has filed lawsuits to the World Trade Organization (WTO) against Slovakia, Poland and Hungary over their bans on food imports from Kyiv.
It said such restrictions were a violation of international obligations by Ukraine's EU neighbours.
They say the bans are needed to protect their farmers from cheap imports.
Russia's full-scale invasion of Ukraine last year all but closed the main Black Sea shipping lanes and forced Ukraine to find alternative overland routes.
That in turn led to large quantities of grain ending up in central Europe.
Farmers in those countries have since held protest rallies, saying that Ukrainian grain shipments were undercutting them and distorting local markets.
The pressure led to the 27-member EU bloc agreeing earlier this year to trade restrictions on Ukraine's imports into Hungary, Poland, Slovakia - as well as Bulgaria and Romania - until 15 September.
On the deadline day, the European Commission - the EU's executive body - decided not to extend the ban.
But the governments in Budapest, Warsaw and Bratislava defied the Commission's move, announcing their own restrictions.
In a statement on Monday, Ukraine's Economy Minister Yulia Svyrydenko said that "it is crucially important for us to prove that individual member states cannot ban imports of Ukrainian goods.
"That is why we are filing lawsuits against them [Slovakia, Poland and Hungary] to the WTO."
Ms Svyrydenko added that Ukrainian exporters had "already suffered and are continuing to suffer significant losses" because of the unilateral bans.
Poland said it would keep its ban regardless.
"We maintain our position, we think it is correct, it results from an economic analysis and powers derived from EU and international law," said government spokesman Piotr Mueller.
"A complaint before the WTO doesn't impress us."
The European Commission has repeatedly stated that it is not up to individual EU members to make trade policy for the bloc.
Poland, Hungary and Slovakia are still allowing Ukrainian grain to transit their countries to other markets.
Poland and Slovakia are key Ukraine's allies in its fight against the Russia invasion - but the grain imports row has caused friction.
Ukraine is one of the world's biggest suppliers of crops such as sunflower oil, wheat, barley and maize.
Russia's full-scale invasion in February 2022 and blockade of Ukraine's Black Sea ports trapped some 20 million tonnes of grain meant for export.
This caused world food prices to soar, and threatened to create shortages in the Middle East and Africa.
A deal was struck in July of that year between Russia and Ukraine to allow cargo ships to sail along a corridor in the Black Sea to Ukrainian ports.
Almost 33 million tonnes of grain were shipped from Ukraine under the deal, and world food prices declined by roughly 20% as a result, according to the UN's Food and Agriculture Organization.
However, Russia pulled out of the deal in July and world grain prices have risen again.