Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty in speech denouncing West

Ukrainian forces continue "to make progress" in counteroffensive efforts against Russian troops along both the northeastern and southern fronts, the U.K. Ministry of Defense said Wednesday in an intelligence update.

In the northeastern Kharkiv region, according to the ministry, Ukrainian forces have "now consolidated a substantial area of territory east of the Oskil River," advancing up to 20 kilometres "beyond the river into Russia's defensive zone towards the supply node of the town of Svatove."

"It is highly likely that Ukraine can now strike the key Svatove-Kremina road with most of its artillery systems, further straining Russia's ability to resupply its units in the east," the ministry said. "Politically, Russian leaders will highly likely be concerned that leading Ukrainian units are now approaching the borders of Luhansk Oblast, which Russia claimed to have formally annexed last Friday."

Russian President Vladimir Putin has signed laws finalizing the illegal annexation of four regions of neighboring Ukraine -- more than 15% of the country's territory -- even as his military struggles to maintain control over the newly absorbed areas.

The documents completing the annexation of Ukraine's Donetsk, Luhansk, Kherson and Zaporizhzhia regions -- in defiance of international laws -- were published on a Russian government website on Wednesday morning.

Earlier this week, the Russian parliament ratified treaties making the occupied areas part of Russia. The move followed what the Kremlin called referendums in the four Ukrainian regions, which the West rejected as a sham.

The annexed areas are not all under control of Russian forces, which are battling a massive counteoffensive effort by Ukrainian troops.

President Joe Biden and Vice President Kamala Harris spoke with Ukrainian President Volodymyr Zelenskyy on Tuesday, underscoring that the U.S. will never recognize areas annexed by President Vladimir Putin as Russian territory and offering additional security assistance.

Biden announced a $625 million security assistance package that includes additional weapons and equipment, according to a statement from the White House.

Biden also promised to impose "severe costs" on any individual, entity or country that "provides support to Russia’s purported annexation."

-ABC News' Justin Gomez

Since Russian President Vladimir Putin announced a national mobilization last month, more than 355,000 people have left the country, according to Russian independent media.

Roughly 200,000 people escaped to Kazakhstan, 80,000 left for Georgia and 65,000 departed for Finland. Some 6,000 people also fled to Mongolia and there are reports of people fleeing to Armenia, Kyrgyzstan, Uzbekistan and Tadjikistan.

Russian Defense Minister Sergei Shoigu said Tuesday that more than 200,000 people have been mobilized since Sept. 21.

-ABC News' Tanya Stukalova