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The town of Sloviansk in Donetsk Oblast will likely become the next "key" battleground in Russia's push to seize the Donbas region of eastern Ukraine, the U.K. Ministry of Defense said Wednesday in an intelligence update.

"Russian forces from the Eastern and Western Groups of Forces are likely now around 16 km north from the town of Sloviansk," the ministry said. "With the town also under threat from the Central and Southern Groups of Forces, there is a realistic possibility that the battle for Sloviansk will be the next key contest in the struggle for the Donbas."

In the meantime, Russian forces likely continue to consolidate control over the town of Lysychansk and the wider Luhansk Oblast, about 45 miles east of Sloviansk.

"To the north, it has committed most of the remaining available units from the Eastern and Western Groups of Forces to the Izium axis," the ministry added. "Over the last week, Russian forces have likely advanced up to another 5 km down the E40 main road from Izium, in the face of extremely determined Ukrainian resistance."

NATO announced on Monday it has concluded negotiations with Sweden and Finland on their accession to the organization.

"Finland and Sweden have completed accession negotiations at the NATO headquarters in Brussels, as agreed last week by the leaders of the countries at the summit in Madrid,” a NATO press release said.

“Both countries have officially confirmed their desire and ability to fulfill their political, legal and military obligations as NATO members," the NATO press service added.

The countries will sign their accession protocols on Tuesday. All member countries will then have to ratify the documents according to their national laws.

Finland and Sweden jointly submitted applications to join NATO on May 18, ending decades of neutrality in the face of Russia's invasion of Ukraine.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Max Uzol and Nataliia Kushnir

Russian President Vladimir Putin celebrated the Russian seizure of Lysychansk and the majority of the Luhansk regional border in eastern Ukraine by appearing to direct his military to conduct an “operational pause”, the Institute for the Study of War reported on Monday.

Putin and Russian Defense Minister Sergey Shoigu called the recent Russian gains in the Luhansk region as a major victory for Russian forces in Ukraine.

The Russian president also stated that the Russian units that participated in the battle for Lysychansk should rest to increase their combat capabilities, the ISW report claimed.

Putin‘s public comment was likely meant to signal his concern for the welfare of his troops in the face of periodic complaints in Russia about the treatment of Russian soldiers, the ISW experts said.

Russian troops that fought through the Luhansk region are very likely in need of a significant period in which to rest and refit before resuming large-scale offensive operations, observers noted.

It is not clear, however, that the Russian military will accept the risks associated with a long enough operational pause to allow these likely exhausted forces to regain their strength, the ISW report said.

Putin was quick to remark on Monday that “other military formations, including the East Group and the West Group, must carry out their tasks according to the previously approved plans.”

“I hope that everything will happen … in the same way as it has happened in Luhansk,” the Russian President added as quoted by local media.

Vyacheslav Volodin, who chairs the Russian Duma -- the lower house of the Federal Assembly -- said on Tuesday that Ukraine was "doing everything" to ensure that Moscow's troops would not stop their "special military operation" at the borders of the self-proclaimed Donetsk People's Republic and Luhansk People's Republic in eastern Ukraine, according to Russian state media.

Serhii Haidai, the head of the Luhansk Regional Military Administration, said on Tuesday that Russian attacks in the Luhansk region destroyed more than 90% of the infrastructure in the territories that were actively defended by the Ukrainian military.

An overwhelming majority of houses were under fire, Haidai said, and most of them “can't be restored.”

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Max Uzol and Nataliia Kushnir

Russian forces will "now almost certainly" switch focus to capturing Donetsk Oblast after claiming control of neighboring Luhansk Oblast in eastern Ukraine's disputed Donbas region, according to intelligence briefings from the U.K. Ministry of Defense.

Russian President Vladimir Putin declared victory in Luhansk on Monday, a day after Ukrainian troops withdrew from the city of Lysychansk -- their last stronghold of resistance in the province. Ukrainian troops had spent weeks trying to defend Lysychansk and to keep it from falling to Russian forces, as the neighboring city of Sievierodonetsk did a week ago. A river separates the two cities.

"Russia's focus will now almost certainly switch to capturing Donetsk Oblast, a large portion of which remain under the control of Ukrainian forces," the U.K. Ministry of Defense said in Monday's briefing. "The fight for the Donbas has been grinding and attritional and this is highly unlikely to change in the coming weeks."

Putin has made capturing the entire Donbas -- Ukraine's mostly Russian-speaking industrial heartland in the east -- a key goal in his war in neighboring Ukraine. Russia-backed separatists in Donbas have battled Ukrainian forces since 2014, when they declared independence from Kyiv after the Russian annexation of Ukraine's Crimean Peninsula. Moscow formally recognized the self-proclaimed republics in Luhansk and Donetsk oblasts just days before launching its Feb. 24 invasion.

"Russia's relatively rapid capture of Lysychansk extends its control across virtually all of the territory of Luhansk Oblast, allowing it to claim substantive progress against the policy objective it presented as the immediate purpose of the war, namely 'liberating' the Donbas," the U.K. Ministry of Defense said in Tuesday's briefing.

"Ukrainian forces have likely largely withdrawn in good order, in line with existing plans," the ministry continued. "The Ukrainian held areas of Sieverodonetsk-Lyschansk consisted of a bulge or salient which Russian could attack from three sides. There is a realistic possibility that Ukrainian forces will now be able to fall back to a more readily defendable, straightened front line."

"The battle for the Donbas has been characterised by slow rates of advance and Russia’s massed employment of artillery, levelling towns and cities in the process," the ministry added. "The fighting in Donetsk Oblast will almost certainly continue in this manner."