Ukraine's president warned against any "appeasement" of Russia, arguing a bilateral deal with Moscow on his country would only make the fighting in eastern Ukraine worse.
He said the new US administration has a "historic chance" to halt Russia's ambitions.
Many in Europe are concerned about the US stance toward Russia under President Donald Trump amid talk of a more cooperative relationship.
Ukrainian president Petro Poroshenko told the Munich Security Conference, however, he had been reassured of Western unity and solidarity earlier in the day by new US Secretary of State Rex Tillerson.
"I hear increasingly obsessive calls for at least some degree of appeasement toward Russia's appetite.
"To move in that direction would be naive, wrong and dangerous, not only for Ukraine, but also for Europe and for the world," he said.
He did not specify who was making those calls but said now is the time to stop Russia's expansionist ambitions.
"This is an absolutely historic chance for all of us, but above all this is a historic chance for the new US administration and solid trans-Atlantic unity," Mr Poroshenko said.
More than 9,800 people have died since April 2014 in fighting in eastern Ukraine between government forces and Russia-backed separatists.
Fighting escalated earlier this month, the worst outbreak since a 2015 peace deal.
On Thursday, Mr Tillerson said Russia must abide by a 2015 peace deal agreed upon in Minsk aimed at ending fighting in eastern Ukraine as the Trump administration searches for ways to work cooperatively with Moscow.
He spoke after meeting with Russian foreign minister Sergey Lavrov at a meeting of Group of 20 foreign ministers in Bonn, Germany.
Mr Poroshenko, however, warned against "any agreement behind our back" with Russia on Ukraine.
"We have no intention to give up" on defending Ukraine, he said, speaking in English.
"Any deal with (Russian President Vladimir) Putin behind Ukraine would only aggravate the situation."
Nato secretary-general Jens Stoltenberg said, following a meeting with Mr Lavrov on the sidelines of the security conference, he again emphasised the need for the Minsk accord to be implemented.
He also said Americans officials had assured him Washington backed Nato.
"The United States remains committed to Nato and to the trans-Atlantic alliance," he told reporters.
In eastern Ukraine, the leader of pro-Russia separatist rebels warned they may use force to drive out Ukrainian troops.
Alexander Zakharchenko, the leader of the self-proclaimed Donetsk People's Republic, said the rebels aim to "free the occupied territories" in the Donetsk and Luhansk provinces by political means, but added that they could do so with military force if political efforts fail.
Mr Putin's spokesman, Dmitry Peskov, said Mr Zakharchenko's statement defied a 2015 peace agreement, but added that it comes amid tensions provoked by Ukraine's actions.