Former German chancellor Gerhard Schroeder on Wednesday said Russia wanted a "negotiated solution" to the war with Ukraine.
But Ukrainian President Volodymyr Zelenskyy dismissed the comments and said any dialogue would be contingent on a Russian ceasefire and withdrawal of its troops.
A friend of Russian President Vladimir Putin and derided in Germany for his pro-Russia stance, Mr Schroeder said last month's agreement on grain shipments from Ukraine, aimed at easing a global food crisis, might offer a way forward.
"The good news is that the Kremlin wants a negotiated solution," Mr Schroeder told Stern weekly and broadcaster RTL/ntv, saying he met Mr Putin in Moscow last week.
"A first success is the grain deal. Perhaps that can be slowly expanded to a ceasefire."
In response, Ukrainian presidential adviser Mykhailo Podolyak called Mr Schroeder a "voice of the Russian royal court" and made it clear that the grain agreement would not lead to broader negotiations.
"If Moscow wants dialogue, the ball is in its court," Mr Podolyak wrote on Twitter. "First, a ceasefire and withdrawal of troops, then constructive [dialogue].
Grain in Ukraine – in pictures
In a video address on Wednesday night, Mr Zelenskyy also responded bitterly to the notion of talks with Russia.
"It is simply disgusting when former leaders of major states with European values work for Russia, which is at war against these values," he said.
Tentative attempts at peace talks in March went nowhere.
Russia is engaged in considerable military activity in the east, north-east and south of Ukraine, the General Staff of the Armed Forces said on Wednesday night.
In the north-eastern region of Kharkiv, where Ukrainian forces have had success in expelling the enemy, Russian forces shelled a dozen towns with tank fire and launched air strikes, the general staff said.
Ukrainian grain on way to Lebanon after clearing inspection - video
Shelling was also carried out near the central town of Kramatorsk, which Russian forces hope to capture as they move south, the military said.
There was shelling in about eight towns and villages, it said.
Russia has started forming a military strike force aimed at Mr Zelenskyy's home town of Kryvyi Rih in central Ukraine, the Ukrainian military said earlier.