In the midst of growing calls to break Russia’s blockade of Ukraine’s Black Sea ports for grain exports, America’s top military general warned that doing so militarily would be a high-risk military operation.

The statements were delivered by Mark Milley, chairman of the United States Joint Chiefs of Staff, to reporters as he came here Tuesday to meet with his “Five Eyes” colleagues before traveling to Finland and Sweden, who have asked for NATO membership, later this week.

Britain, Canada, Australia, and New Zealand are members of the Five Eyes information sharing cooperation. The discussions take place as Russia battles for control of the Donbass industrial region and as its blockade of Odesa, Ukraine’s primary grain export port, exacerbates a worldwide food crisis.

“You can take the grain out by truck or train, or you can take it out by sea. Right now, the sea lanes are blocked by mines and the Russian navy. In order to open up those sea lanes would require a very significant military effort,” Milley said. If policymakers opted for it, “it would be a high-risk military operation that would require significant levels of effort.”

US President Joe Biden has pledged to protect NATO and has coordinated arms shipments to Ukraine, but he has resisted other requests for US soldiers to intervene in order to prevent direct battle with Russia and triggering a broader war. Biden voted against a no-fly zone requested by Ukrainian President Volodymyr Zelenskyy in March because he saw it as escalatory.

Retired Adm. James Stavridis, a former NATO supreme allied commander, proposed this week that ships under the aegis of the United States, NATO, or a coalition of nations might guard grain convoys in the same way that US naval vessels protected oil supplies during tensions with Iran in the 1980s.

Amid reports that up to 20 million tons of grain are stuck inside Ukraine — the world’s fifth-largest producer of wheat — Army Gen. Christopher Cavoli, the nominee for top US general for operations in Europe, praised Germany’s rail agency last week for enabling transport of Ukraine’s grain over land into Western Europe via its “Berlin train lift,” a play on the Berlin airlift in the aftermath of World War II.

Last week, the White House rejected Russian President Vladimir Putin’s offer to expedite grain and fertilizer shipments in exchange for the West lifting sanctions against Moscow for the Ukraine conflict.

Milley is in Europe ahead of NATO’s summit this summer to express solidarity for friends who have united against Russia’s incursion, as well as to assist coordinate military supplies for Ukraine ahead of a third conference of donor nations held by Defense Secretary Lloyd Austin on June 15.

“We have to make sure we have continued, coordinated synchronized support to the Ukrainians, both lethal and non-lethal,” Milley said.

Three months into the battle, Milley acknowledged that there are differences of opinion among allies about what success against Russia would look like. Zelenskyy has frequently stated that he wants the Russians driven back to where they were on February 23, before the large-scale invasion began, but he has also stated that he is willing to talk with Moscow.

“I think that agreement might be too strong a term,” Milley said in response to a question. “But generally speaking, most of the policies of the various countries would say that the rules-based order must remain intact, Ukraine must remain a free and sovereign country.”