Ukraine’s ministry of renovation and infrastructure says the U.N.-brokered Black Sea grain export deal has been halted again because Russia has blocked registration of ships to all Ukrainian ports.

A U.N. spokesman said Russia had informed officials overseeing the initiative that Moscow would limit registrations to the port of Pivdennyi, in Ukraine’s Odesa province, until all parties agree to unblock the transit of Russian ammonia.

The United Nations and Turkey brokered the Black Sea Grain Initiative between Moscow and Kyiv last July to help tackle a global food crisis aggravated by Russia’s full-scale invasion of Ukraine.

Russia agreed in May to a two-month extension of the deal, which covers three Ukrainian ports including Pivdennyi, but has said the initiative will cease unless an agreement aimed at overcoming obstacles to Russian grain and fertilizer exports is fulfilled.