By EMMA SIMPSON

Yara International, which operates in more than 60 countries, buys considerable amounts of essential raw materials from Russia.

Fertiliser prices were already high due to soaring wholesale gas prices.

Yara's boss, Svein Tore Holsether, has warned the situation could get even tougher.

"Things are changing by the hour," he told the BBC.

"We were already in a difficult situation before the war... and now it's additional disruption to the supply chains and we're getting close to the most important part of this season for the Northern hemisphere, where a lot of fertiliser needs to move on and that will quite likely be impacted."

Russia and Ukraine are some of the biggest producers in agriculture and food globally.

Russia also produces enormous amounts of nutrients, like potash and phosphate - key ingredients in fertilisers, which enable plants and crops to grow.

"Half the world's population gets food as a result of fertilisers... and if that's removed from the field for some crops, [the yield] will drop by 50%," Mr Holsether said.

"For me, it's not whether we are moving into a global food crisis - it's how large the crisis will be."

Svein Tore Holsether, chief executive of Yara International.

His company has already been affected by the conflict after a missile hit Yara's office in Kyiv. The 11 staff were unharmed.

The Norwegian-based company isn't directly affected by sanctions against Russia, but is having to deal with the fall-out. Trying to secure deliveries has become more difficult due to disruption in the shipping industry.

Just hours after Mr Holsether spoke to the BBC, the Russian government urged its producers to halt fertiliser exports.

He pointed out that about a quarter of the key nutrients used in European food production come from Russia.

"At the same time we're doing whatever we can do at the moment to also find additional sources. But with such short timelines it's limited," he said before the news emerged.

Analysts have also warned that the move would mean higher costs for farmers and lower crop yields. That could feed through into even higher costs for food. (BBC)