Lukashenko, who met Putin in Moscow for more than five hours, did not say what the proposal entailed but added that it had been outlined to US President Donald Trump when he held a summit with Putin in Alaska last month.
"President Putin and I discussed it but I won't talk about it. The president himself will say," Lukashenko said.
"It's a good proposal for Ukraine, proposals that were heard by Donald Trump in Alaska, among other places, and taken to Washington for consideration and discussion. A very good proposal," Lukashenko told Russian TV reporter Pavel Zarubin.
"If the Ukrainians don't accept these proposals, it will be like it was at the beginning of the special military operation," he added, using Russia's term for the invasion of Ukraine.
"It will be even worse; they will lose Ukraine."
Russia has previously insisted on terms that Ukraine rejects as tantamount to surrender, including the handover of more territory, the renunciation of Ukraine's NATO membership ambitions and the imposition of limits on the size of its armed forces.
Lukashenko was speaking three days after Trump, in an unexpected shift, said Ukraine was capable of regaining all its lost territory - amounting to nearly a fifth of the country - and should act now because Russia's economy was in deep trouble.
The Kremlin rejected those comments, attributing them to the fact that Trump had just met Ukrainian President Volodymyr Zelenskiy and saying he had been influenced by Zelenskiy's view of the conflict.
Zelenskiy has said that Putin is pretending to negotiate but has no real interest in peace.
"To avoid losing all of Ukraine, (Zelenskiy) must not just negotiate but agree to favourable terms - terms that, by and large, have been approved by the Americans," Lukashenko said.
He also suggested that he, Putin and Zelenskiy, as leaders of three Slavic states, should sit down and come to an agreement.
Lukashenko meets Putin more often than any other foreign leader and has backed his war in Ukraine, although without committing his own troops to fight.
He has been isolated for years by sanctions over his war role and human rights record.
But Trump in recent weeks has phoned him, praised him as a "highly respected" leader and sent an envoy to Belarus for talks that led to the freeing of more than 50 political prisoners.
US officials say the administration hopes to draw Belarus out of Russia's geopolitical orbit, even if only to a degree.
But some political analysts see this as unlikely, given Lukashenko's dependence - politically, economically and militarily - on his much larger neighbour.
Countries on the European Union's east have agreed on the need for a "drone wall" with advanced detection, tracking and interception capabilities, EU Defence Commissioner Andrius Kubilius said on Friday.
Drones play a major role in the war in Ukraine, with both sides boosting their domestic production, while increased drone activity has exacerbated security concerns in several EU member states in recent weeks.
"Russia is testing the EU and NATO and our response must be firm, united, and immediate," Kubilius told reporters in Finland after a videoconference with defence ministers mostly from the EU's east and representatives of Ukraine and the NATO military alliance.
Participants have agreed to move from "discussions to concrete actions" and that the planned "drone wall" will be part of a broader Eastern Flank Watch that will also include ground defences and maritime security, he said.
Kubilius said EU heads of state and government were expected to hold "intensive" discussions on the issue in October.
Representatives from Bulgaria, Estonia, Finland, Latvia, Lithuania, Poland, Romania, Hungary, Slovakia and Denmark attended the video conference.
Establishing a network of detectors must be a first priority, the commissioner said, adding that surveillance systems should be integrated and capabilities in areas such as jamming and drone interception improved.
