Ukraine expects a "maximum escalation" from Russia in the coming months as the Kremlin on Wednesday backed bonuses for troops who hit NATO weapons.
Members of the Western alliance are considering more support for Ukraine as the almost year-long war enters a crucial phase ahead of the winter thaw.
Oleksiy Danilov, secretary of the National Security and Defence Council of Ukraine, told Britain's Sky News that he expected about half of the more than 320,000 soldiers mobilized by Russia late last year would form part of a renewed attack.
"I'm conscious the main fights are yet to come and they will happen this year, within two to three months. These will be defining months in the war," Danilov said.
"Russia is preparing for maximum escalation," he added. "It is gathering everything possible, doing drills and training.
"Those countries who help us in our struggle have started to provide us maximum help."
German Defence Minister Boris Pistorius visited the armed forces in the western state of North Rhine Westphalia on Wednesday to learn more about the Leopard 2 main battle tank, 14 of which are being sent to Ukraine. Germany currently opposes sending fighter jets.
Spain meanwhile wants to supply Ukraine with four to six Leopard tanks, the newspaper El País reported.
The vice president of the German parliament, Katrin Göring-Eckardt, said on a visit to Kiev that she understood Urkaine's wish for speedy deliveries.
"It is important for the Ukrainian government that the delivery of the Leopard tanks takes place quickly, because President Vladimir Putin is obviously preparing an offensive," she told dpa after talks with Ukrainian Prime Minister Denys Shmyhal.
The head of the Munich Security Conference, Christoph Heusgen, warned against a hasty rejection of Ukraine's arms demands.
"This rush to say no should finally stop, it is harmful," Heusgen told German media.
But the dangers of NATO becoming more involved in the conflict were spelt out by Kremlin spokesman Dmitry Peskov saying that bonuses could be paid to Russian soldiers for hitting weapons in Ukraine sent from abroad.
"As for these tanks, we have already said that they will burn. Of course, even more so if there are such incentives," Peskov said.
He added that Russia doubts the continuation of the last major disarmament treaty with the US - but at the same time stressed its importance.
"There are few indications of that," he was quoted as saying by the Interfax agency when asked whether further talks on extending the New Start treaty beyond 2026 were on the horizon.
"But we consider the continuation of this treaty very important."
The US is supplying Kiev with Abrams tanks.
The German government has meanwhile reacted cautiously to Brazilian President Luiz Inacio Lula da Silva's proposal for possible mediation between Russia and Ukraine.
"Now we have to see if anything can come out of it," government spokesman Steffen Hebestreit said.
