The remarks came as Ukraine gave its first estimate of the death toll its armed forces have suffering in the battles of nearly 9,000 soldiers killed. The UN separately said that 5,587 civilians killed in conflict since Russia launched a "special military operation" against its neighbour on February 24.
Fearing attacks targeting its 31st independence day on Wednesday, local authorities in Kyiv have banned large public events, rallies and other gatherings related to the anniversary from Monday until Thursday due to the possibility of rocket attacks.
Turkish President Recep Tayyip Erdogan used a visit to Ukraine last week to plead for revived peace talks after almost six months of fighting, building on the Black Sea grain agreement brokered by the UN and Turkey. Mr Gatilov said the UN had become too politicised to mediate peace talks effectively and said contacts between Russia, Ukraine and the western powers backing Mr Zelenskyy had dried up.
"Now, I do not see any possibility for diplomatic contacts. And the more the conflict goes on, the more difficult it will be to have a diplomatic solution," he said.
"We do not have any contacts with the western delegations. On the protocol side we do not see each other. Privately we do not have any contacts, unfortunately... we simply do not talk to each other."
Peace talks in the early weeks of the war appeared to show some signs of progress, with Ukraine offering to discuss a form of neutrality in which it would stay out of Nato — meeting one of the Kremlin's pre-war demands.
But the talks came to a halt after evidence emerged of possible Russian atrocities near Kyiv, and Mr Zelenskyy has repeatedly said Russia will shut down the diplomatic route if it keeps crossing red lines.
He said in his nightly address on Sunday that no further talks would be possible if Russia parades captured Ukrainian fighters at a "show trial" in Mariupol, where Ukraine says a makeshift court is being set up in a concert hall.
"If this despicable show trial takes place, if our people are brought into this scenery in violation of all agreements, all international rules, if there is abuse... This will be the line beyond which any negotiations are impossible," he said.
Ukraine crisis - in pictures
Mr Gatilov said the non-existent peace talks made it impossible to say how long the war would last, as Russia ignores continued pleas from western powers to withdraw from Ukraine.
Ukraine said on Monday that Russia was firing from tanks and artillery weapons near Kharkiv and preparing for possible Ukrainian counter-offensives in the south and east.
A regular British intelligence update said Russia was having difficulty recruiting auxiliary forces to aid its military campaign in the eastern Donbas region.
The British Defence Ministry said commanders were probably resorting to "direct financial incentives" to bring in more troops, but that some combat units were deemed too unreliable to be sent into attack.
Underlying this problem is the fact the Kremlin refuses to call its invasion a war and insists it is a "special military operation", limiting the Russian state's legal powers, the ministry said.