Russia now controls over 90% of eastern Ukraine's Luhansk Oblast and "is likely to complete control in the next two weeks," the U.K. Ministry of Defense said Friday in an intelligence update.

"Russia is now achieving tactical success in the Donbas. Russian forces have generated and maintained momentum and currently appear to hold the initiative over Ukrainian opposition," the ministry said. "Russia has achieved these recent tactical successes at significant resource cost, and by concentrating force and fires on a single part of the overall campaign."

Russia has been unable to advance its other fronts or axes, "all of which have transitioned to the defensive," according to the ministry. In fact, the ministry noted, none of the strategic objectives of Moscow's original plan have been achieved.

Russian forces failed to achieve their initial objectives to seize Ukraine's capital, Kyiv, and other major cities.

"Staunch Ukrainian resistance and the failure to secure Hostomel airfield in the first 24 hours led to Russian offensive operations being repulsed," the ministry said. "Following the failure of the initial plan, through false planning assumptions and poor tactical execution, Russia adapted its operational design to focus on the Donbas."

Russian forces are now battling Ukrainian troops for control of the Donbas region of eastern Ukraine, which comprises Luhansk and Donetsk oblasts.

"In order for Russia to achieve any form of success will require continued huge investment of manpower and equipment, and is likely to take considerable further time," the ministry added.

Friday marked the 100th day since Russian forces invaded neighboring Ukraine.

In a statement, the Ukrainian Ministry of Foreign Affairs described the "full-scale invasion" as a "continuation of Russia’s aggressive actions it unleashed 8 years ago by occupying Crimea and parts of territories of Donetsk and Luhansk regions."

"Today, our people defend not only their right to exist and live in an independent state, but the security of Europe and the whole democratic world," the ministry said. "Every day our defenders, at the cost of their own lives, bravely repel Russia’s war machine and fight for freedom and peaceful future of the continent."

"For 100 days of war, the Kremlin has failed to reach its main goal -- conquest of Ukraine," the ministry added. "Instead, Russia has become the most sanctioned state in the world, and its activities within international organizations and participation in international events have been significantly limited or stopped. The Ukrainian army is bravely holding the line and has liberated territories in a number of regions. Ukraine is determined to have a complete victory over the Russian invader."

The ministry thanked the "dozens of countries around the world who provide significant support" to Ukraine. It also called on the international community to support the establishment of a special tribunal to investigate alleged war crimes committed by Russian forces in Ukraine.

"In order to stop Russia's crimes against the Ukrainian people, destruction of our economy and blackmail of the whole world by famine, consistent support for Ukraine should continue. Assistance to our state today is the best investment in peace and sustainable development of all mankind," the ministry said. "The main pillars for our victory remain unchanged: maximum sanction pressure on Russia, deliveries of necessary weapons and granting Ukraine the status of candidate on the way to full-fledged EU membership."

Meanwhile, Russian Ministry of Foreign Affairs spokesperson Maria Zakharova applauded progress that's been made in the so-called special military operation in Ukraine.

"The special military operation will be continued until all of its objectives declared by the Russian administration, including denazification and demilitarization of Ukraine, are achieved," Zakharova said at a press briefing Friday. "A lot has been done in this area: militants of the Azov nationalistic formation have surrendered in Mariupol and the liberation of Donbas has been consistently carried out."

Russia now controls over 46,300 square miles of Ukraine, which accounts for about 20% of Ukraine's territory, according to Ukrainian President Volodymyr Zelenskyy.

Since Russia's invasion began, Russian forces have gained control of over 16,602 square miles, or roughly 7% of Ukraine's territory -- an area that's comparable to the size of the Netherlands, Zelenskyy said Thursday in a speech to the Chamber of Deputies of Luxembourg. Combined with the territory from Russia's annexation of Crimea in 2014 and the war in the Donbas region, Russia's control of Ukraine now accounts for 20% of its territory, he said.

-ABC News' Christine Theodorou

Since the start of Russia's invasion, the Ukrainian first lady has been in hiding with her two children. A difficult question her 9-year-old son keeps asking is when the war will end, Olena Zelenska said in an exclusive interview with ABC News.

"Unfortunately, I don't think any Ukrainian would be able to answer that question," Zelenska told "Good Morning America" co-anchor Robin Roberts in her first televised solo interview since the invasion began.

In discussing the state of the conflict nearly 100 days after Russian President Vladimir Putin launched a "special military operation" in Ukraine, Zelenska said that conceding territory to Russia won't stop the war.

"You just can't concede ... parts of your territory. It's like conceding a freedom," Zelenska, 44, said in the interview, airing on "Good Morning America" Thursday. "Even if we would consider territories, the aggressor would not stop at that. He would continue pressing, he would continue launching more and more steps forward, more and more attacks against our territory."

The interview with Zelenska will air Thursday on "Good Morning America" and across ABC News. "Good Morning America" airs at 7 a.m. on ABC.

Russia's invasion of Ukraine has already engaged around 330,000 Russian personnel, Brigadier General Oleksiy Hromo of the General Staff of the Ukrainian Armed Forces told local media on Thursday.

“[Russian] groups at various axes are close to 150,000 strong. If we add their air and sea components, there are about 220,000 military personnel,” Hromo said. Russia also deployed “units of the national army combat reserve, the federal service of guard forces, and mobilization units,” the Brigadier General added.

Hromo's sobering account came on the back of claims made by a top Ukrainian official earlier on Wednesday that revealed mounting Ukrainian casualties. Up to 1,000 Ukrainian soldiers are being killed or wounded each day in the Donbas region of eastern Ukraine, with 200 to 500 killed on average and many more wounded, said David Arakhamia, who heads the presidential faction in the Ukrainian parliament.

In early June, Ukrainian President Volodymyr Zelenskyy said daily fatalities among Ukrainian ranks at the Donbas front were between 60 to 100 troops.

Arakhamia, one of Zelenskyy's closest advisers who oversees Ukraine's stalled negotiations with Russia, has been leading a Ukrainian delegation in Washington this week in a bid to lobby the Biden administration and Congress.

Arakhamia's team want to see the pace of weapons shipments increased and is pushing for a designation of Russia as a state sponsor of terrorism.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Yulia Drozd and Max Uzol