Russia's envoy to the United Nations in Geneva on Wednesday said "no real progress" had been achieved in resolving issues raised by Moscow over the Black Sea grain deal, which is set to expire next month.
Separately, Ukraine accused Russia of blocking the passage of four grain ships from its Black Sea ports.
The Kremlin has repeatedly said the deal will not be renewed beyond May 18 unless the West removes obstacles to Russian grain and fertiliser exports.
"As of today, we regret to say that no real progress was reached in resolving this problem," envoy Gennady Gatilov told reporters at the Russian permanent mission in Geneva.
The Black Sea deal, brokered between Russia and Ukraine by the United Nations and Turkey in July, aimed to prevent a global food crisis by allowing Ukrainian grain trapped by Russia's invasion to be safely exported from three ports.
The Ukrainian navy, writing on Facebook, said Russia had effectively stalled the deal on Wednesday by refusing to take part in agreeing upon the routes that four grain ships should take, a process that occurs for each export trip.
Although Russia's agricultural exports have not been explicitly targeted by Western restrictions, Moscow says sanctions on its payments, logistics and insurance industries have created a barrier to the export of its grains and fertilisers.
"The Ukrainian part of the deal is working well," Gatilov said. "But the Russian part of this deal is not working and these two parts should be equal. This is not the case."
One of Russia's main demands in negotiations is the reconnection of the Russian Agricultural Bank (Rosselkhozbank) to the SWIFT payments system.
Gatilov said the bank should be fully reconnected to the system rather than subjected to "case-by-case" decisions.
Reuters cited a Russian source as saying the United States gave JPMorgan Chase & Co permission to process payments for Russian Agricultural Bank, but that the arrangement was no substitute for reconnecting the bank to SWIFT.
(Additional reporting by David Ljunggren; Editing by Barbara Lewis and Cynthia Osterman)
