Iran and six world powers — the United States, France, Germany, China, Russia and Britain — aim to end a decade-old nuclear standoff by a self-imposed Nov. 24 deadline.
The talks are centered on curbing Iran's atomic activities in exchange for a lifting of sanctions hurting its economy.
But with less than six weeks to go before this target date, Western officials say there are still significant differences between the sides, especially over the future scope of Iran's uranium enrichment production.
One of Iran's chief negotiators, Deputy Foreign Minister Abbas Araqchi, last week raised the possibility that the talks could be extended. But a U.S. State Department official said Washington believed there was still time to reach a comprehensive solution by the target date.
"I'm sure that a compromise is possible," said Lavrov, during a visit to Paris where he met U.S. Secretary of State John Kerry on Tuesday.
"I can't guarantee you that it would be reached by Nov. 24. This date is not sacred," he told Russian television. "We are striving to reach a result before this date, but I'm sure that the main thing is not artificial schedules but the essence of the agreements. That is the main thing for us."
Lavrov himself will not be in Geneva for Wednesday's talks.
Iran rejects Western allegations that it is seeking nuclear weapons capability, but has refused to halt uranium enrichment, and has been hit with U.S., EU and UN Security Council sanctions as a result.
Iranian Foreign Minister Mohammad Javad Zarif said the sides "might need more time" to discuss the issues and potential solutions, Iran's ISNA news agency reported on Wednesday.
"We are reviewing all the possible solutions to end the disputes. The fact that there are eye-catching disputes, does not mean they cannot be resolved," it quoted Zarif as saying after meeting Ashton in Vienna on Tuesday, where they will hold talks with Kerry on Wednesday.
"We have not reached a common conclusion yet, but I think it can be reached if there is a political will," he added.