Russian fertilisers and agricultural products must be able to reach world markets "unimpeded" or a global food crisis could strike as early as next year, UN Secretary General Antonio Guterres said Saturday.

"It is important that all governments and the private sector cooperate to bring them to market," he said from the Joint Coordination Center.

The JCC oversees the implementation of the Ukrainian grain export agreement signed in July by Kyiv and Moscow with the United Nations and Turkey as guarantors.

The agreement also guarantees Russia the right to export its agricultural products and fertilisers despite Western sanctions.

"What we see here in Istanbul and in Odessa is only the more visible part of the solution. The other part of this package deal is the unimpeded access to the global markets of Russian food and fertiliser, which are not subject to sanctions," Guterres said, adding that despite this, Russian fertiliser and agricultural exports still faced "obstacles."

"Without fertiliser in 2022, there may not be enough food in 2023. Getting more food and fertiliser out of Ukraine and Russia is crucial to further calm commodity markets and lower prices for consumers," he said.

Guterres travelled this week to Ukraine, where he met the presidents of Ukraine and Turkey, Volodymyr Zelensky and Recep Tayyip Erdogan, in the western city of Lviv Thursday.

He headed to the southern city of Odessa on Friday.

Earlier Saturday, he visited the first aid ship chartered by the United Nations to transport Ukrainian grain on the southern shores of Istanbul in the Sea of Marmara.

The Brave Commander left the Ukrainian port of Pivdenny on Tuesday with 23,000 tonnes of wheat before crossing the Bosphorus on Wednesday evening.

The UN chief vowed Thursday that his organisation would try to "step up" grain exports from Ukraine before the onset of winter, as they are crucial for food supplies in many African countries.

In Ireland, meanwhile, the Panama-flagged Navi Star carrying 33,000 tonnes of grain, arrived two weeks after having left southern Ukraine's main port of Odessa -- one of the first to leave following July agreement.

Ukraine's Ambassador to Ireland, Larysa Gerasko said the continuation of shipments ultimately "depends on Russia, on Russian actions", she added.

"We expect Russia to keep to its commitments under the Black Sea grain initiative."

Ships must use a safe corridor to travel in the Black Sea and then be inspected by the JCC before being allowed to cross the Bosphorus Strait.

Cereal exports from Ukraine, one of the world's leading producers and exporters, were blocked for several months due to the Russian invasion of Ukraine in February, raising fears of a global food crisis.