As Sweden and Finland consider NATO memberships, British Prime Minister Boris Johnson signed new security pacts with the leaders of both nations on Wednesday, ensuring the U.K. could give military support even if the countries are not NATO members.

Johnson said the invasion of Ukraine "sadly" opened a new chapter.

"Our armed forces will train, operate and exercise together -- marrying our defense and security capabilities and formalizing a pledge that we will always come to one another’s aid," he said at a news conference.

-ABC News' Christine Theodorou

The Ukrainian prosecutor’s office has brought its first indictment against a Russian soldier for allegedly shooting and killing a Ukrainian civilian.

Vadim Shishimarinov, a 21-year-old soldier in Russia's 4th Armored Division, allegedly fired his AK-47 at a car driven by a 62-year-old Ukrainian man, killing him at the scene, Ukraine's prosecutor general Iryna Venediktova said.

He could face a life sentence if convicted of the shooting, which unfolded on Feb. 28 in the village of Chupakhivka.

-ABC News' Tomek Rolski

Fighting has continued on Ukraine's Zmiinyi Island, also known as Snake Island, "with Russia repeatedly trying to reinforce its exposed garrison located there," the U.K. Ministry of Defense said Wednesday in an intelligence update.

"If Russia consolidates its position on Zmiinyi Island with strategic air defence and coastal defence cruise missiles, they could dominate the north-western Black Sea," the ministry warned.

The island is strategically located, as it provides access to Ukraine's major port cities, including Odesa, the country's largest seaport and one of the largest in the Black Sea basin, serving as a major gateway for grain shipments and other key exports.

Ukraine, however, has managed to successfully strike Russian air defenses and resupply vessels with drones, according to the ministry.

"Russia's resupply vessels have minimum protection in the western Black Sea, following the Russian Navy’s retreat to Crimea after the loss of the Moskva," the ministry added.

Ukrainian President Volodymyr Zelenskyy on Wednesday thanked the U.S. House of Representatives for its approval of a nearly $40 billion package of humanitarian and military aid to Ukraine in a bill passed on Tuesday.

“We are looking forward to the consideration of this important document for us by the Senate,” Zelenskyy wrote in a tweet expressing gratitude to House Speaker Nancy Pelosi, President Joe Biden and “all friends of Ukraine” in the House.

Earlier on Tuesday, two senators unveiled a resolution calling on the Biden administration to designate Russia as a state sponsor of terrorism. Sen. Richard Blumenthal, D.-Conn., and Sen. Lindsey Graham, R.-S.C., said the designation, which could result in further sanctions for Russia, is warranted by Russia's war on Ukraine and conduct elsewhere under President Vladimir Putin.

“Putin is a thug, and a bully, and he will continue being an increasing threat to Europe and the world unless he is stopped,” Blumenthal said at a press conference, adding, “If there's anybody who embodies terrorism, totalitarianism and tyranny, it's Putin.”

The resolution was originally requested by the Ukrainian parliament, but Secretary of State Antony Blinken has so far declined to add Russia to the list of countries -- Cuba, Iran, North Korea and Syria -- considered to be state sponsors.

Russia's Ministry of Health sent the World Health Organization a letter in which it "strongly rejects" its responsibility for the rising death toll in Ukraine.

“The health emergency in Ukraine and neighboring countries is caused by the unconstitutional coup that took place in this country in 2014, and by the subsequent military aggression of the Kyiv regime against the population of Donbass in the period of 2014–2022,” Andrey Plutnitsky, Russia's envoy to the WHO, wrote in the letter.

Plutnitsky called on WHO member states to pay “special attention” to “the military biological activities undertaken by a number of countries on the territory of Ukraine, which represent a significant danger to the region.”

Members of the Russian military are trying to hide their involvement in some hostilities in Ukraine, the Ukrainian Defense Ministry's intelligence service said on Wednesday.

Russian troops, including some from the air and space forces, have demanded that the command keep their involvement in the invasion secret in order to avoid further possible responsibility for war crimes, the Ukrainian intelligence claimed.

Meanwhile, the U.S. Department of Defense confirmed that 89 of the 90 M777 Howitzers promised to Ukraine have been handed over to the Ukrainian military.

Of the total count of 184,000 rounds of ammunition committed to Ukraine, “65 percent, so almost 120,000 rounds, are in [Ukraine],” a senior defense official said at a background briefing on Tuesday. The Russians added two battalion tactical groups to their force in Ukraine in the past 24 hours, the official added, bringing the new count of operational Russian BGTs up to 99.

-ABC News' Edward Szekeres, Max Uzol, Yuriy Zaliznyak and Irene Hnatiuk

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