Secretary of State Marco Rubio said Wednesday that an expected minerals deal would give the United States a "vested interest" in Ukraine's security, although he stopped short again of promising formal guarantees.
"I wouldn't couch it as a security guarantee, but certainly, if the United States has a vested economic interest that's generating revenue for our people as well as for the people of Ukraine, we'd have a vested interest in protecting it," Rubio told reporters on a refuelling stop in Ireland.
"Certainly one of the things that provides for Ukraine's long-term prosperity and security is vibrant economic growth and development," he said.
A growing economy "gives them a tremendous amount of leverage and power and the ability to fund their own defences," he said.
Ukrainian officials agreed in talks with Rubio and President Donald Trump's national security advisor Mike Waltz in Jeddah on Tuesday to back a 30-day ceasefire.
The two sides also agreed to sign as soon as possible a minerals deal giving the United States access to Ukraine's natural wealth.
The deal went unsigned after a disastrous White House meeting on February 28 between Trump and President Volodymyr Zelensky.
Rubio said that the United States was initiating contact with Russia on Wednesday and was gauging its response.
"That's what we want to know -- if they're prepared to do it unconditionally," Rubio said of the Russian response.
"If the response is, 'yes', then we know we've made real progress, and there's a real chance of peace. If their response is 'no', it would be highly unfortunate, and it'll make their intentions clear."
Trump has sharply shifted the US stance by negotiating with Russia and demanding concessions from Ukraine, stunning European allies.
Rubio said Europeans would have to be involved eventually in discussions if they progress as Russia will be keen to end European sanctions.
Russia claims sweeping gains in Kursk region
Russia has made sweeping gains in its Kursk region in the past 24 hours, taking back control of five villages previously held by Ukrainian troops, the Russian military said.
Moscow's forces have also entered Sudzha, the largest town that fell under Kyiv's control, and were conducting "assault operations" there, the state TASS news agency reported.
Kyiv launched its surprise assault on the Kursk region in August, but has steadily lost ground as Moscow deploys thousands of reinforcements including North Korean soldiers.
Ukraine now risks losing its grip on the border region entirely, ceding dozens of square kilometres of territory in the past six days, according to military bloggers.
Russian army units "liberated the settlements of Kazachya Loknya, 1st Knyazhy, 2nd Knyazhy, Zamostye and Mirny" in the Kursk region, all on the outskirts of Sudzha, the Russian defence ministry said Wednesday.
"The information provided by our military shows that our troops are successfully advancing in the Kursk Region, liberating areas that were under the control of the militants. The dynamic is good," Kremlin spokesman Dmitry Peskov told reporters.
Video shared by Russian news outlets on Wednesday purported to show Russian troops waving a flag in the centre of Sudzha, which Kyiv captured shortly after its offensive began.
Kursk was one of Kyiv's few bargaining chips in swapping land with Russia, which has seized and occupied around a fifth of Ukraine since it took Crimea in 2014 and launched its full-scale assault in February 2022.
In a village east of the fighting, a Ukrainian attack on an agricultural plant killed four people, acting Kursk region governor Alexander Khinshtein said Wednesday.