Russia has "lost momentum and fallen significantly behind schedule" in its offensive on eastern Ukraine's Donbas region, the U.K. Ministry of Defense said.

"Despite small-scale initial advances, Russia has failed to achieve substantial territorial gains over the past month whilst sustaining consistently high levels of attrition," the ministry said on Sunday. "Russia has now likely suffered losses of one third of the ground combat force it committed in February."

Russian forces in late March pulled out of the suburbs north of Kyiv and collected in eastern Ukraine. They began an offensive in April, but have since foundered, the U.K. said.

Low troop morale, reduced combat effectiveness and loss of equipment have all played a role in slowing the Russian advance, the ministry said.

Russian Foreign Minister Sergei Lavrov said the West has declared a "total hybrid war" on Russia and warned that everyone will feel the consequences on Saturday.

"The collective West has declared a "total hybrid war" against us and it is hard to predict how long it will last, but it is clear that everyone, without exception, will feel the consequences," he said at a meeting of the Council for Foreign and Defense Policy on Saturday.

Lavrov added: "We did our best to avoid a direct clash, but if they have challenged us, we, of course, accept it," the minister said. "We are used to sanctions, they have existed nearly forever in one or another kind."

In a phone call initiated by Finland on Saturday, Russian President Vladimir Putin and Finnish President Sauli Niinisto discussed Finland’s decision to join NATO.

Putin described the decision as a mistake that could damage relations between their two countries, according to the Kremlin.

"Putin stressed that abandoning the traditional policy of military neutrality would be wrong, since there are no threats to Finland's security. Such a change in the country's foreign policy course may negatively affect Russian-Finnish relations, which for many years have been built in the spirit of good-neighbourliness and partnership, and have been mutually beneficial," the Kremlin said.

Niinisto's office said Finland initiated the call.

""President Niinisto told President Putin how fundamentally the Russian demands in late 2021 aiming at preventing countries from joining NATO and Russia’s massive invasion of Ukraine in February 2022 have altered the security environment of Finland," Niinisto's office said in a statement.

Niinisto told Putin that every independent nation maximizes its security, according to Niinisto's office.

“The conversation was direct and straight-forward and it was conducted without aggravations. Avoiding tensions was considered important," President Niinisto says.

-ABC News' Somayeh Malekian

U.S. senators, including Senate Minority Leader Mitch McConnell visited Kyiv, Ukrainian President Volodymyr Zelenskyy announced in a video posted online Saturday.

“The visit of the U.S. Senate delegation led by the leader of the Republican minority in the upper house of Congress Mitchell McConnell is a strong signal of bipartisan support for Ukraine from the United States Congress and the American people," Zelenskyy said on Telegram.

Zelenskyy added: "Thank you for your leadership in helping us fight not only for our country, but also for democratic values and freedoms. We really appreciate it.”

-ABC News' Clack Bentson

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn