On the campaign trail, Donald Trump vowed to quickly end the war in Ukraine. He’d do it in “24 hours” after taking the oath, he said, or even before his inauguration.  But as he prepares to return to the White House, it’s clear that promise will go unfulfilled.  Nearly three years after Russia launched a full-scale invasion of Ukraine, there’s no end in sight to the war, Europe’s worst since World War II. Fighting rages across a long front line, with Russian forces waging a grinding offensive in Ukraine’s east and Ukrainian troops holding on to territory inside Russia in the Kursk region, where North Korean soldiers have arrived to bolster Moscow’s ranks. Russia has suffered large casualties, with an estimated 700,000 dead or wounded, according to U.S. and British officials. Since Trump’s election victory, his team has not outlined a peace proposal to Ukraine’s leadership, according to two sources close to Ukraine’s government and a former U.S. diplomat. And there has been no shuttle diplomacy between Kyiv, Moscow and Mar-a-Lago.  Members of Trump’s chosen national security team have in recent weeks acknowledged the difficulties of brokering a possible peace accord. “Let’s set it at 100 days and move all the way back and figure a way we can do this in the near term to make sure that the solution is solid, it’s sustainable, and that this war ends so that we stop the carnage,” retired Lt. Gen. Keith Kellogg, Trump’s pick to serve as special envoy to Ukraine, told Fox News last month.  Sen. Marco Rubio, R-Fla., Trump’s choice for secretary of state, told senators at his confirmation hearing on Wednesday that forging a peace deal between Ukraine and Russia will be “very difficult.”  “This will not be easy,” Rubio said. “Conflicts of this nature that have historical underpinnings to it are going to require a lot of hard diplomacy and tough work, but that’s something that needs to happen.” Kellogg is expected to travel to Ukraine for talks soon after Trump’s inauguration on Monday, sources familiar with the matter said. Kellogg had tentative plans to travel to Ukraine earlier but chose to postpone the trip, the sources said.  Trump’s transition team did not respond to a request for comment. Although Trump’s team has worked with the Biden White House on securing the Israel-Hamas ceasefire and hostage release deal, there has been no such collaboration on Ukraine, according to the sources. Rep. Mike Waltz, R-Fla., whom Trump has tapped to serve as his national security adviser, has held several conversations on Ukraine with President Joe Biden’s national security adviser, Jake Sullivan, NBC News previously reported. The talks have focused on sharing information but have not explored strategies for ending the war or securing a ceasefire. Trump has offered no details as to how he envisions ending the conflict, apart from taking advantage of his personal relationships with Russian President Vladimir Putin and Ukrainian President Volodymyr Zelenskyy.  Trump has suggested that U.S. military aid to Ukraine may be reduced in his administration, that European governments should increase their support for Ukraine, and that he disagreed with Ukraine firing longer-range missiles into Russian territory. Trump, whose fraught relationship with Zelenskyy’s government in his first term led to his first impeachment, has also suggested the war is primarily a European problem. Protests 59 mins ago A rebranded Women's March returns before Trump's inauguration Donald Trump 7 hours ago Trump's inauguration ceremony moved indoors due to frigid weather forecast Immigration 4 hours ago Senate advances a migrant detention bill that could be Trump's first law to sign “War with Russia is more important for Europe than it is for us. We have a little thing called an ocean in between us,” he told NBC News’ Kristen Welker in December.  Trump’s recent comments have raised concerns in Ukraine and among its allies in Washington that the incoming administration might push Ukraine to make painful concessions without putting pressure on Russia, according to Western diplomats, former U.S. officials and analysts. Regional experts and Western diplomats are skeptical that Putin would be ready to make concessions in negotiations when his forces are slowly but steadily gaining ground in eastern Ukraine. But Kellogg has said Trump will not force Ukraine to swallow a bad deal. “I think what people need to understand — he’s not trying to give something to Putin or to the Russians. He’s actually trying to save Ukraine and save their sovereignty, and he’s going to make sure that it’s equitable and that it’s fair,” Kellogg said this month. Kellogg co-wrote a report last year calling for pressuring both Moscow and Kyiv to enter into peace talks. Under the plan, Washington would threaten more military aid to Ukraine if Russia failed to negotiate while offering possible relief from economic sanctions if Moscow accepted a settlement. At the same time, Washington would threaten Kyiv with cutting off assistance if it refused to enter into talks. NATO membership for Ukraine would be ruled out for an extended period, but foreign powers would provide security guarantees for Kyiv. Russia has criticized elements of Kellogg’s proposal and similar ideas floated in Washington as unacceptable. Moscow has insisted that Ukraine be a permanently neutral state with strict limits imposed on its military, and that Kyiv give up territory held by Russian forces as well as areas still controlled by Ukrainian troops. But Russian Foreign Minister Sergei Lavrov said Tuesday his country is open to talks with Trump. Lavrov also praised Trump for saying he believes NATO’s willingness to eventually accept Ukraine as a member of the alliance helped cause the war. Lavrov also praised Trump’s incoming national security adviser, Waltz, for saying that it was unrealistic that Ukraine could regain all the territory it had lost in the war. “The very fact that people have increasingly started to mention the realities on the ground deserves welcome,” Lavrov said.  Ukraine will most likely come under increasing pressure over its mobilization rules in order to ensure continued U.S. support. At the moment, Ukrainians under age 25 are not subject to conscription. But Waltz said recently that Ukraine should lower the country’s draft age to get more troops into the fight and help stabilize the front line. “When we hear about morale problems, when we hear about issues on the front line, look, if the Ukrainians have asked the entire world to be all in for democracy, we need them to be all in for democracy,” Waltz told ABC News on Sunday. Sullivan, Biden’s national security adviser, also suggested in an interview with Bloomberg that Kyiv will need to alter its mobilization policy, saying the Ukrainian government will need to address its manpower shortage. NBC News’ Katherine Doyle and Olympia Sonnier contributed. This story first appeared on NBCNews.com. More from NBC News: How Trump’s inauguration guests reflect his new popularity this time Democrats push for inaugural oversight as Trump committee rakes in cash Biden administration says it will leave TikTok ban enforcement for Trump