Russia is staging helicopters, artillery systems and troops in preparation for what is expected to be a renewed offensive in the Donbas region of eastern Ukraine, a senior U.S. defense official said Wednesday.

Russian forces have been using the cities of Belgorod and Valuyki in Russia near Ukraine's northeast border as primary sites to stage equipment and resupply troops, the official said. The United States is now seeing a third Russian town, Rovenki, also near the Ukraine northeast border, being used for that purpose.

The official said there's already signs that Russian forces are on the move south to the Donbas region.

"We continue to see units flowing into the northern Luhansk Oblast, that north part of the Donbas," the official said. "They're flowing in from Valuyki and from that town called Rovenki."

The long Russian convoy is heading south and, at last check, was near the city of Izium in eastern Ukraine, according to the official.

Other Russian troops to the south of Izium appear to be working to improve their mobility and firepower in the region, the official said.

"We've seen them try to erect a temporary bridge over a local river," the official said. "They're increasing their artillery in the area."

-ABC News' Matt Seyler

In a brief moment of joy amidst the brutality of war, rescuers in eastern Ukraine on Wednesday pulled a puppy alive from the rubble of a bombed building, authorities said.

The rescue unfolded in Mykhailivka in the Donetsk region, according to the Donetsk Regional Police.

Police released a video showing rescuers digging through the rubble with bare hands to reach the trapped pooch. Rescuers said they heard the puppy whining as they were picking through the rubble.

"Thanks to the boys for doing everything quickly and promptly here," said the dog’s owner while holding the trembling puppy in his arms

Finland and Sweden -- both traditionally militarily neutral countries -- are considering a dramatic pivot in their security policy following Russia's invasion of Ukraine.

Leaders of both countries publicly stated during a joint press conference Wednesday that they are considering taking steps to join the NATO alliance.

"The European security architecture has changed fundamentally after Russia's invasion of Ukraine," Finland's Prime Minister Sanna Marin said. "The change in the security landscape makes it necessary to analyze how we best secure peace for Finland and in our region in the future."

Swedish Prime Minister Magdalena Andersson added, "We have to really think through what is best for Sweden and our security and our peace in this new situation and, of course, what is happening and the discussion in Finland is important for us to follow. Therefore, we need to have a very close contact, but we have to have a process in Sweden to think this through."

Last week, NATO Secretary-General Jens Stoltenberg said he's had close contact with political leaders of both countries and has conveyed that it's up to them whether to decide joining NATO.

"But if they apply, I expect that 30 allies will welcome them and that we will find ways to also address the concerns they may have about this interim period between (when) they have applied and until the last ratifications has taken place," Stoltenberg said.

Kremlin spokesperson Dmitry Peskov warned that further expansion of NATO to include Finland and Sweden will not contribute to security in Europe.

"In itself, the alliance is rather a tool sharpened for confrontation, this is not an alliance that ensures peace and stability," Peskov said, according to Russian state-run news agency TASS.

-ABC News' Christine Theodorou

About 1.4 million people have been left without clean running water in war-torn eastern Ukraine and an estimated 4.6 million people across the country are at risk of losing their supply, the United Nations Children's Fund reported Wednesday.

UNICEF officials said heavy fighting in eastern Ukraine, including the widespread use of explosive weapons in populated areas, has decimated a large part of the region's water systems. The agency tallied 20 separate incidents in which water infrastructure has been damaged or destroyed and warned of a "risk of complete collapse."

Damaged electrical grids have shut down water pumps and explosion-related damage to pipelines are disrupting the flow of water, according to UNICEF.

"Water is essential for life and a right for everyone," Osnat Lubrani, the U.N. resident coordinator in Ukraine, said in a statement. "The health risks, particularly for children and the elderly, caused by water stoppages are severe, as people are forced to use dirty water sources, resulting in diarrhoea and other deadly infectious diseases."

Murat Sahin, a UNICEF Ukraine representative, added that, “Young children who live in conflict zones are 20 times more likely to die from diarrheal diseases linked to unsafe water than from direct violence, as a result of war."

In hard-hit Mariupol, which has been under siege since the start of the Russian invasion on Feb. 24, thousands of residents trapped in the city are seeking any water they can find and resorting to dirty water sources, according to UNICEF. Major cities across the Donetsk and Luhansk regions are also cut off from water supplies.

The water systems in Sumy, Chernihiv and Kharkiv have also been seriously damaged, UNICEF said. An additional 340,000 people are at risk of losing their water supply from a reservoir in Horlivka in the Donetsk region that is inching closer to running dry, according to UNICEF.

Agency officials said that prior to the invasion, much of the water systems in eastern Ukraine were already ailing after eight years of a low-grade conflict in the region.

-ABC News' Christine Theodorou

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan