"I am concerned, and we are working hard in order to make sure that it will be possible to maintain the Black Sea Initiative and at the same time that we are able to go on in our work to facilitate Russian exports," Guterres told a press conference.
That crucial accord granting safe passage for Ukrainian grain to be exported via Black Sea ports - despite Moscow's invasion - was signed in July 2022 by Russia, Ukraine, Turkey and the United Nations. It was renewed again in May but for only two months, until July 17.
Moscow is demanding guarantees on another agreement concerning its own exports, in particular of fertilizer components before moving forward on renewing the grain initiative.
Russia said last week that a Ukrainian sabotage group had blown up a section of a pipeline that Russia used to export ammonia - a key component in making fertilizer.
That stretch of the conduit is in Ukraine. The two countries have blamed each other for the attack.
The resumption of Russia's ammonia exports through the pipeline is one of Moscow's conditions to continue with the grain export deal.