WARSAW, Poland (AP) — Experts are working to come up with new export routes for millions of tons of grain stuck war-torn Ukraine but it will take time, a Polish government official said Monday.

Vast amounts of corn and other grains are stuck in Ukraine – one of the world’s largest grain producers – and cannot be exported because Russia has blocked the country’s ports.

Michal Dworczyk, the head of the Polish prime minister’s office said two major border crossings have been expanded and dedicated to cargo traffic only, while plans are in the works to standardize Ukraine’s tracks with European ones and to increase the export capacity of Poland’s sea ports.

“We are facing a very big challenge,” Dworczyk said. “We need to say it honestly, you do not solve such problems from one day to another… from one week to another.”

Dworczyk said that Poland was pleased with U.S. President Joe Biden’s offer earlier this month to build temporary grain silos along Ukraine’s borders, including with Poland. He said experts need to ascertain the type and location of the silos.

The idea is for grain to be transferred to the silos and then transported by vehicle to ports for export.