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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"Ukrainian forces have likely largely withdrawn in good order, in line with existing plans," the ministry continued.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."

Speaking of the situation in Ukraine in an exclusive interview with Reuters over the weekend, Pope Francis said he might be heading to Ukraine after returning from his trip to Canada.

Francis, who has repeatedly condemned Russia's invasion of Ukraine, said he “would like to go [to Ukraine],” but wanted to “go to Moscow first."

No pope has ever visited Moscow. Last Thursday, Francis implicitly accused Russia of waging a "cruel and senseless war of aggression."

Francis noted over the weekend that there have been contacts between Vatican Secretary of State Cardinal Pietro Parolin and Russian Foreign Minister Sergei Lavrov about a possible trip to Moscow.

“After I come back from Canada, it is possible that I will manage to go to Ukraine," Francis said. "The first thing is to go to Russia to try to help in some way, but I would like to go to both capitals."

According to Francis, the Vatican first inquired about a trip to Russia several months ago, but Moscow said it was not the right time.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Max Uzol and Fidel Pavlenko

Ukraine spends almost as much on its army in a month as it did in a year before Russia's invasion, Ukrainian Prime Minister Denys Shmyhal told local media on Sunday.

Ukraine's monthly military budget is now about $4.4 billion, according to Shmyhal. Before the war, its annual military budget was close to $5.3 billion, the prime minister said.

Shmyhal added that Ukraine's overall monthly budget is lacking around $150 million. Many businesses have suspended work or completely ceased to exist, he said, adding that millions of tons of harvest remain stuck in warehouses due to Russia blocking the path for exports.

On Sunday, Turkish customs detained a Russian cargo ship with grain stolen by Russian forces and shipped from the Ukrainian port of Berdyansk, the Ukrainian ambassador to Turkey said.

Turkish President Recep Tayyip Erdogan said Friday that Turkey will be able to re-export grain products, including wheat, oat, and barley from the Black Sea to countries in need after talks with Russia and Ukraine that will be held in the coming days.

Turkey has about 20 bulker ships ready to transport grain, Erdogan added.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Max Uzol and Fidel Pavlenko

Ukrainian troops have retreated from the eastern Ukrainian city of Lysychansk in order to save the lives of soldiers, Ukrainian military officials said Sunday.

The General Staff of Ukraine's Armed Forces confirmed that Ukrainian troops have left Lysychansk after weeks of heavy fighting.

Ukrainian military officials said a combination of Russian artillery superiority, the prowess of Russian military aviation and its rocket salvo systems proved too much for Ukranian troops.

"This was done to save the lives of the soldiers," the general staff said in a statement, adding that the continuation of the defense of the city "would lead to fatal consequences."

Russian forces now effectively control Luhansk Oblast, bringing them a step closer to seizing Donbas region of eastern Ukraine.