The "BSGI" grain deal, which has helped ease the global food crisis caused by the invasion, will automatically renew on March 18 unless Moscow or Kyiv object.
But Russian Foreign Minister Sergei Lavrov said Thursday that extending the deal was becoming "complicated", as he claimed a parallel agreement on Russian exports was not being respected.
While the focus is the export of Ukrainian grain, the second agreement, between Moscow and the UN, aimed to facilitate the export of Russian food and fertilizers, which are exempt from Western sanctions imposed on Moscow.
"If the package is half fulfilled, then the issue of extension becomes quite complicated," Lavrov said during a press conference in Moscow.
"Our Western colleagues, the United States and the European Union, pathetically declare... that no sanctions apply to food and fertilizers, but this position is dishonest," Lavrov said.
"In fact the sanctions prohibit Russian ships carrying grain and fertilizers from entering the corresponding ports, sanctions prohibit foreign ships from entering Russian ports to pick up this cargo," he said.
More than 23.7 million tons have been exported under the UN and Turkey-brokered Black Sea Grain Initiative, according to the United Nations.
- 'Too much at stake' -
Russia's foreign ministry spokeswoman Maria Zakharova said Thursday that a Russian "interdepartmental delegation" will head to Geneva for the talks.
She said the discussions would include UN humanitarian chief Martin Griffiths, and Rebeca Grynspan, head of the UN's trade and development agency UNCTAD.
"The deal will be discussed," the spokeswoman told reporters.
The UN confirmed talks were scheduled for Monday.
UNCTAD said the deal had allowed for greater consistency and predictability, helping to mitigate price shocks in the international market.
"The BSGI is one example of concrete action to navigate some of the worst cost-of-living crises the world has faced in a generation," it said Thursday, in a report on the deal's impact.
"The BSGI has yielded results that need to be scaled up. There is too much at stake and the situation is dire. Without the initiative, the lives of millions more are placed in a precarious position.
"The renewal of the BSGI provides hope that the world's most vulnerable can make it through the crises. And every effort is needed to keep this hope alive."
- Corn and wheat -
On Tuesday, UN chief Antonio Guterres said it was crucial to extend the deal, during a visit to Kyiv. Grynspan was in the Ukrainian capital on Wednesday for talks on the BSGI.
Guterres' deputy spokesman Farhan Haq told reporters in New York that Grynspan had been "pushing very hard" to make sure that obstructions to Russian fertilizer exports are cleared.
The BSGI accounted for 60 percent of total Ukrainian export volumes of corn, wheat and barley during the first four months of its operation, said UNCTAD.
Nearly half of the exports are corn, and more than a quarter wheat.
Around 45 percent of the exports went to developed countries. The biggest recipient was China, followed by Spain, Turkey, Italy and the Netherlands.