Russian President Vladimir Putin's "special military operation" into neighboring Ukraine began on Feb. 24, with Russian forces invading from Belarus, to the north, and Russia, to the east. Ukrainian troops have offered "stiff resistance," according to U.S. officials.
The Russian military has since launched a full-scale ground offensive in eastern Ukraine's disputed Donbas region, capturing the strategic port city of Mariupol and securing a coastal corridor to the Moscow-annexed Crimean Peninsula.
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Kremlin spokesperson addresses missing Americans, Brittney Griner
Kremlin spokesperson Dmitry Peskov told MSNBC that Americans Andy Tai Ngoc Huynh and Alexander Drueke, who were captured while fighting in Ukraine, "committed crimes," and that they were not part of the Ukrainian armed forces and therefore not subject to the Geneva Conventions.
"They were involved in firing and shelling our military personnel, they were endangering their life and they should be responsible. They should be held responsible … for those crimes that they have committed," he said.
The Geneva Conventions outline the humanitarian rights given to prisoners of war, however, mercenaries are not given the same protections.
Regarding WNBA star Brittney Griner, who has been detained in Russia since February, Peskov said she’s not a hostage.
He said Russia has strict drug laws and she was caught carrying banned substances.
Griner was taken into custody at an airport near Moscow after officials allegedly found vape cartridges with hashish oil in her bag. Hasish oil is illegal to possess in Russia. The U.S. government has classified her case as "wrongfully detained," which means that the U.S. would work to negotiate her release.
Ben Stiller, a goodwill ambassador with UNHCR, visits Ukraine
Actor Ben Stiller, a goodwill ambassador for the United Nations High Commissioner for Refugees for the last five years, is visiting Ukraine to highlight the refugee crisis.
"I’m here meeting people forced to flee their homes due to the war in Ukraine. People have shared stories about how the war has changed their lives -- how they’ve lost everything and are deeply worried about their future," Stiller said Monday from Ukraine.
"Protecting people forced to flee is a collective global responsibility," he said. "We have to remember this could happen to anyone, anywhere."
Stiller was seen meeting with Ukrainian President Volodymyr Zelenskyy on Monday. Stiller told the actor-turned-president, "What you’ve done and the way that you’ve rallied the country and for the world, it’s really inspiring."
Stiller also met with displaced people in Poland.
-ABC News' Christine Theodorou
Russians launching large-scale offensive in Luhansk region
Serhiy Haidai, the head of the Regional Military Administration in Luhansk in eastern Ukraine, said the situation along the entire Luhansk front is "extremely" difficult with Russian forces "launching a large-scale offensive in our region."
"They have accumulated a sufficient number of reserves and today all the free settlements of the region are on fire," Haidai said.
The city of Lysychansk in the Luhansk Oblast has been coming under "massive" Russian fire all day, he said, with the number of victims unknown. He said Russian forces are advancing along the Lysychansk-Bakhmut highway and nearby settlements are under constant fire.
Haidai added that Ukrainian troops are only in control of the Azot chemical plant in Severodonetsk.
'Historic week' begins for Ukraine, Zelenskyy says
Monday marks the beginning of a "truly historic week" for Ukraine, as the country awaits a decision on its future within the European Union, Ukrainian President Volodymyr Zelenskyy said during his Sunday evening address.
"We will hear the answer from the European Union on the candidate status for Ukraine," Zelenskyy said. Last week, the European Commission backed Ukraine for EU candidate status. Now it is up to the European Council to confirm Ukraine's status, with a decision expected by the end of this week, the Ukrainian president said.
"I am convinced that only a positive decision meets the interests of the whole of Europe," Zelenskyy said.
He added that Ukraine -- and other European countries -- should expect increased hostility from Russia in the coming week.
"We are preparing. We are ready. We warn partners," he said.
But as combat units from both sides of the conflict remain committed to intense combat in the Donbas, they are likely experiencing dips in morale, the United Kingdom's Ministry of Defense stated in a Sunday intelligence update.
"Ukrainian forces have likely suffered desertions in recent weeks, however, Russian morale highly likely remains especially troubled," the ministry said.
As cases of whole Russian units refusing to carry out orders and armed stand-offs between officers and their troops continue to occur, Russian authorities are likely struggling to put legal pressure on the dissenters due to the invasion's official status as a 'special military operation', the UK report said.
Low Russian morale is driven by "perceived poor leadership, limited opportunity for rotation of units out of combat, very heavy casualties, combat stress, continued poor logistics, and problems with pay," according to the Defense Ministry. Many Russian personnel of all ranks also likely remain confused about the war's objectives, it said.
The U.K. Defense Ministry also said Monday the struggles of Russia's air force likely contributed to the exhaustion of Russian ground troops. "In the conflict to date, Russia's air force has underperformed," another intelligence update said on Monday.
"Its failure to consistently deliver air power is likely one of the most important factors behind Russia's very limited campaign success," the report stated. Despite boasting relatively modern and capable combat jets, Russia's air combat training has for years highly likely been heavily scripted and designed to impress senior officials, as opposed to fostering modern skill-sets, the Defense Ministry said.
As a result, Russian ground troops in Ukraine are becoming worn out while a heavy reliance on advanced cruise missiles has likely led to their stocks running low, the report concluded.
More shelters, less music
The Ukrainian Parliament on Sunday supported a bill on the construction of a network of bomb shelters across Ukraine, including in new buildings.
"The war has shown that there were few reliable shelters in Ukraine," said Olena Shulyak, a member of parliament.
Many of the existing shelters are not equipped with evacuation exits, lack access to water supply and sewerage systems, and are not adapted for food storage, Shulyak said on Telegram, adding, "Not to mention their ability to protect the population in the event of weapons of mass destruction."
The parliament topped off a busy weekend when it banned music by artists with Russian citizenship from being aired in public and in Ukrainian media to prevent the influence of "separatist sentiment in the population," according to the new bill.
A two-thirds majority of lawmakers agreed that Russian music would make the adoption of a Russian identity more attractive while weakening the Ukrainian state.
-ABC News' Edward Szekeres, Yuriy Zaliznyak, Max Uzol and Yulia Drozd