Russia would face a “large scale” war if it invaded and tried to take the city of Kharkiv, warned Ukrainian President Volodymyr Zelenskiy.
US and Russian diplomats made no major breakthrough during negotiations on Ukraine but agreed to keep talking to try to resolve a crisis that has stoked fears of a military conflict.
After talks in Geneva, US Secretary of State Antony Blinken warned of a "swift, severe" response if Russia invades Ukraine after massing troops near its border.
At the same time Russian Foreign Minister Sergei Lavrov said Moscow was still waiting for a written response to its demands for security guarantees.
The industrial city of Kharkiv has a large Russian speaking population among the 1.4 million people living there and Zelenskiy believes it is a “feasible” target for Russian President Vladimir Putin.
He told the Washington Post : "I will say realistically if Russia decides to enhance their escalation, of course they are going to do this on those territories where historically there are people who used to have family links to Russia.
"Kharkiv, which is under Ukraine government control, could be occupied. Russia needs a pretext: they will say that they are protecting the Russian-speaking population."
He pointed out: "And if this will be war, it’s going to be a very strong war, and everyone will lose. Hundreds of thousands of lives will be lost. Ukraine will suffer; Russia will suffer; European countries adjacent to Ukraine will suffer; and the ones who are further away will be impacted by a migration crisis. For us, the most important thing is to preserve our territory."
In the negotiations between the US and Russia on Friday, Blinken did see grounds to hope that mutual security worries could be overcome.
"Based on the conversations we've had - the extensive conversations - over the past week and today here in Geneva I think there are grounds for and a means to address some of the mutual concerns that we have about security," Blinken said.
He described the talks as "frank and substantive" and said Russia now faced a choice.
"It can choose the path of diplomacy that can lead to peace and security, or the path that will lead only to conflict, severe consequences and international condemnation," said Blinken.
"We've been clear - if any Russian military forces move across Ukraine's border, that's a renewed invasion. It will be met with swift, severe and a united response from the United States and our partners and allies."
Lavrov claimed the talks were open and useful and that Moscow would understand whether talks were on the right track once it had received a written response to its sweeping security demands from the United States.
Russia's demands include a halt to NATO's eastward expansion and a pledge that Ukraine will never be allowed to join the Western military alliance.
"I can't tell you if we're on the right track or the wrong track. We'll understand this when we receive the American response on paper to all the points in our proposal," Lavrov said.
Tatiana Stanovaya, head of political analysis firm R.Politik, commented on Telegram: "This is partly a trap, of course, because any such written response will be used to discredit the US negotiating position."
Blinken said he expected to share with Russia "our concerns and ideas in more detail and in writing next week" and said he and Lavrov had "agreed to further discussions after that".
Russia and the United States could hold another meeting next month to discuss Moscow's demands for security guarantees, Russia's RIA news agency quoted a source in the Russian delegation as saying.
Lavrov said Russia had worries of its own, "not about invented threats, but real facts that no one hides - pumping Ukraine with weapons, sending hundreds of western military instructors".
Asked about the possibility of a summit between Putin and US President Joe Biden, Lavrov was circumspect.
"Let's not get ahead of ourselves, President Putin is always ready for contacts with President Biden, it's clear these contacts need to be seriously prepared," he said.
Blinken said of another potential meeting between Biden and Putin: "If we conclude, and the Russians conclude, that the best way to resolve things is through a further conversation between them, we're certainly prepared do that."
