"This war is honestly about one part of Ukraine called Donetsk. The Russians occupy all but about 11% of that area," Rubio told Breitbart News.
He explained that Ukraine considers the territory essential to its national defence because surrendering it could leave the route towards Kyiv exposed.
"In that province, or in that region of Donetsk, the Ukrainians have a defensive line that they've embedded themselves in. They argue - and military strategists say is true - that it is the last defensible line that if they give that up, there is nothing to defend Kyiv," Rubio said.
According to the Secretary of State, Russia has made full control of Donetsk a condition for declaring victory, while Ukraine believes conceding the remaining territory would create an unacceptable military risk.
"The core of this conflict is that Russia demands all of Donetsk, including that defensive line as part of their territory. That's what they've defined as victory," he said. "And Ukraine has said we can't give up that 11% that's remaining of Donetsk because that defensive line is our last defensible line."
Rubio acknowledged that several other disputes remain unresolved but described the territorial standoff in Donetsk as the central obstacle to a negotiated settlement.
Russia targets Ukraine's energy infrastructure
Rubio said Russian forces had been unable to capture the remaining Ukrainian-held territory through battlefield advances and were instead attempting to increase pressure on the country's civilian population.
"The Russians have not been able to advance on the battlefield and take that region," he said.
He said Moscow was using drone and ballistic missile attacks to damage Ukraine's energy infrastructure, including its electricity generation capacity, particularly as winter approaches.
"So what they are doing is they are applying extraordinary pain on the people of Ukraine through drone strikes, through ballistic missile strikes, trying to destroy their energy infrastructure, their electricity, the ability of Ukraine to generate electricity, especially with winter coming up," Rubio said.
Ukraine has responded by targeting Russian oil and gas refineries, which Rubio described as central to financing Moscow's economy and war effort. He said Ukrainian strikes had brought the conflict deeper into Russian territory, with explosions being reported in cities including Moscow and St. Petersburg.
"For the first time, Russians are seeing explosions going on in Moscow, in St. Petersburg deep into their territory. It's no longer a farway war," Rubio said.
The secretary said the United States remained prepared to mediate between Russia and Ukraine if an opportunity emerged, although the entrenched positions of both sides made negotiations difficult.
"The United States remains willing, able, and prepared to play a mediating role if the opportunity presents itself to end this war through a negotiated settlement, which is how we think this war will ultimately end," he said.
Rubio acknowledged that previous diplomatic efforts had not succeeded despite considerable time invested by US officials, including Jared Kushner and Steve Witkoff. He said Washington would continue exploring ways to end the conflict but also had other global priorities requiring attention.
"Not an easy one to negotiate and not an easy one to fix," he said, describing the positions of Moscow and Kyiv as deeply entrenched. "We'd like the war to end, but ultimately we also have other priorities around the world we're focused on as well."