Russia-Ukraine updates: US sanctions Russian military shipbuilder, diamond miner

More than 4 million people have been forced to flee Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the United Nations Refugee Agency.

The tally from the U.N. High Commissioner for Refugees (UNHCR) amounts to just over 9% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in 35 days.

More than half of the refugees crossed into neighboring Poland, UNHCR figures show.

-ABC News' Zoe Magee

The United States is warning that Russian authorities "may single out and detain U.S. citizens" in both Russia and Ukraine.

The warning came Tuesday as the U.S. Department of State issued new travel advisories for the two warring countries.

The State Department previously warned Americans in Russia that they could be targets for harassment by Russian authorities. But the latest advisory makes it explicit that U.S. citizens could be "singled out," "including for detention."

The State Department has also previously warned Americans against traveling to Ukraine to join the fight against Russian forces, pointing to statements from Russian authorities that anyone detained while fighting will not be considered a lawful combatant. That could mean mistreatment or worse, according to State Department spokesperson Ned Price.

"There are continued reports of U.S. citizens being singled out and detained by the Russian military in Ukraine and when evacuating by land through Russia-occupied territory or to Russia or Belarus," the latest advisory for Ukraine states.

Both Russia and Ukraine have been on the State Department's "Travel Advisory Level 4 - Do Not Travel" for months, as tensions ratcheted up and with little to no diplomatic presences on the ground.

-ABC News' Conor Finnegan

In his latest national address, Ukrainian President Volodymyr Zelenskyy urged Ukrainians to stay vigilant and warned that Russia still has "significant potential" to continue its attacks.

The comments come after Russia claimed it would move forces away from Kyiv and Chernihiv and the latest round of in-person peace negotiations kicked off in Turkey.

Zelenskyy said there were positive signals that came out of Tuesday's talks in Istanbul and that Ukraine is willing to continue the negotiation process, but that his country's sovereignty and territorial integrity must be guaranteed.

"There can be no compromise on sovereignty and our territorial integrity. And there will not be any," he said.

The president called for sanctions targeting Russia to be intensified amid continued shelling.

"The question of [lifting Russian] sanctions cannot even be raised until the war is over, until we get back what's ours and until we restore justice," he said.

-ABC News' Fidel Pavlenko

Ukraine outlined proposals for a new system of security guarantees for their country during talks with Russia in Turkey on Tuesday, officials said.

Ukraine is seeking security guarantees comparable to NATO's collective defense clause, Article 5, in that in the event of an attack on Ukraine, "guarantor countries" would be legally obligated to provide arms and impose a "no-fly" zone over Ukraine, according to David Arakhamia, the head of Ukraine's negotiation team. The security guarantees would "not work temporarily" in the currently occupied territories of the Donetsk and Luhansk regions and Crimea, the government said.

The suggested list of guarantors includes permanent members of the UN Security Council -- the U.S., Great Britain, France, China and Russia -- as well as Turkey, Germany, Canada, Italy, Poland and Israel.

In exchange, Ukraine's would "undertake not to deploy foreign military bases, foreign military contingents on its territory, not to join military-political alliances, and military exercises on the territory of Ukraine will be possible with the consent of the guarantor countries," the government said.

It is "fundamentally important" that a potential treaty does not deny Ukraine's right to join the European Union, Oleksandr Chalyi, a member of the Ukrainian delegation, said in a statement.

Negotiations with Russia will continue in the next two weeks, and Ukraine has already begun consultations with all countries they would like to see on the guarantor list, according to Chalyi.

-ABC News' Christine Theodorou