The BSGI deal, which has helped ease the global food crisis caused by the invasion, will automatically renew on Mar 18 unless Moscow or Kyiv object.
But the Kremlin claims that a parallel agreement on Russian exports is not being respected.
While the BSGI concerns the export of Ukrainian grain, the second agreement, between Moscow and the UN, is aimed to facilitate the export of Russian food and fertilisers, which are exempt from Western sanctions imposed on Moscow.
Last Tuesday, UN chief Antonio Guterres said during a visit to Kyiv that it was crucial to extend the deal.
Grynspan was in the Ukrainian capital last Wednesday for talks on the BSGI.
Nearly half of the exports shipped under the deal are corn and more than a quarter are wheat, according to UN data.
Around 45 per cent of the exports went to developed countries. The biggest recipient was China, followed by Spain, Turkey, Italy and the Netherlands.