A strike by Polish truckers on the border with Ukraine threatened to spread with a group of farmers planning to join the protest at a fourth crossing in a dispute that may put humanitarian aid to a war-stricken country at risk.

The blockade, which began two weeks ago, has left as many as 20,000 vehicles stuck on either side of the border and prompted authorities in Kyiv to offer help to evacuate stranded drivers. 

Truckers from Poland and several of the European Union’s eastern member states have argued that temporary concessions put in place in the aftermath of Russia’s invasion have created unfair competition from Ukraine and want them scrapped.

Current tensions are reminiscent of a conflict over grain shipments from Ukraine that led Poland and Hungary to put in place unilateral import bans earlier this year. They come as Ukraine already struggles with a stalling offensive and faces doubts over additional financial aid from the EU and the US.    

About 100 farmers from Betrayed Countryside association are planning to stage a three-day protest at the Medyka crossing on Thursday, Lukasz Martyn, one of its organizers, told Bloomberg. “Farmers were the first group to suffer,” he said. “Now there are transportation companies, the question is who will be next.”

Members of the International Road Transport Union, an industry group, from Hungary, Poland, Slovakia, the Czech Republic and Lithuania have called in a letter to European Commission President Ursula von der Leyen to review an agreement that allows traffic liberalization with Ukraine, which expires in June 2024. 

The EU’s executive said Thursday that any re-introduction of permits or quotas for road transport from Ukraine isn’t legally possible as it would violate the current agreement between the EU and Kyiv. 

The EU allowed Ukrainian carriers to move goods without obtaining permits for an easier and faster flow of shipments during the war.

Ukrainian Deputy Infrastructure Minister Serhiy Derkach said on his Facebook account Monday, following a visit to the Rava-Ruska and Krakivets checkpoints, that there were no more than 15 protesters at the two crossings, effectively holding the border hostage. 

“It’s clear that assurances on unimpeded access for humanitarian aid and dangerous cargo aren’t fulfilled,” according to Derkach. “We have seen numerous instance of fuel cisterns and humanitarian aid queuing. We have passed this information to Polish colleagues and are waiting for a response.”

The current protest comes as pro-EU opposition parties in Poland prepare to take power after they won a parliamentary majority in the Oct. 15 election. Outgoing Infrastructure Minister Andrzej Adamczyk said he has urged Ukraine to open additional lines on their side of the border to ease traffic.