When a 28-year-old volunteer named Nikolai stepped onto a sandy beach on Russia’s Black Sea coast in a hazmat suit just before New Year’s Eve, he was so overwhelmed by the amount of thick oil film that he almost broke down.

He and other volunteers were tasked with shoveling away the oil-drenched sand, but “the scale is just too big,” he said.

Two weeks into the new year, and four weeks after the spill, President Vladimir V. Putin acknowledged the extent of the disaster and dispatched senior officials to deal with Russia’s largest oil spill in years, which has befouled some of the country’s most popular beaches.

The oil was released by two aging Russian tankers that were damaged during a heavy storm in the Kerch Strait on Dec. 15. At least 2,400 metric tons of oil spewed into the sea, Russian officials said.

The disaster in the strait, which separates the Crimean Peninsula from mainland Russia, raised questions about whether the vessels were part of the so-called shadow fleet that Moscow uses to evade sanctions on its oil industry, sometimes employing ships in shoddy conditions.

One of the vessels, the Volgoneft-212, split in half and sank, killing one crew member. The other, the Volgoneft-239, ran aground near the port of Taman. The two vessels were loaded with a total of 9,000 tons of heavy fuel oil, and the authorities are now working not only to clean up the shores, but also to try to contain additional spills from the ship that ran aground.

Russian officials originally claimed that the spill was contained, but soon after the disaster, sightings of floating oil and tarred birds were reported all along Russia’s Black Sea coast.

On Thursday, Mr. Putin ordered a report on the condition of Russia’s tanker fleet, and also asked a deputy prime minister to review Russian legislation covering oil shipments by sea and river and to look into the “scientific advances on the cleanup of similar disasters,” his press office said.

Last week, the Ukrainian Navy warned that oil from the spill could reach Ukraine’s Black Sea coast near Odesa and Mykolaiv, but Ukraine’s Environmental Ministry said a day later that it saw no immediate threat.

Nikolai was among hundreds of volunteers who have lent a hand to the cleanup. A Moscow entrepreneur, he had viewed information from photographs and videos posted by local residents and officials, and traveled to the resort town of Anapa as the new year approached.

In a phone interview with The New York Times after he returned home, he said he had spent a week shoveling away the oil that was washing onto the shore. He asked that his surname not be used because he fears he might lose out on state contracts.

Individuals and businesses chipped in to provide some volunteers with hazmat suits and some basic equipment, but the task was daunting.

“I had seen the photos before I arrived,” Nikolai said. “Yes, it looked bad — but it’s different when you see it in real life. You take the shovel and scoop out that black blob of oil, and it feels like just a drop in the ocean.”

The air along the coast was so heavy with oil fumes, Nikolai said, that he felt dizzy and weak after walking there without a respirator.

Cleanup teams have been responding to oil spills along a coastline of almost 500 miles, collecting over 160,000 tons of contaminated sand and soil as well as 25 tons of “oil-containing liquid,” Russia’s Emergency Situations Ministry said this week.

But the spill risks becoming a “long-term environmental disaster,” according to Greenpeace Ukraine, which criticized the slow Russian response and warned of a deadly impact on Black Sea marine life.

Environmentalists say the spill is particularly difficult to clean up because of the tankers’ cargo. Heavy fuel oil, unlike ordinary regular crude oil, does not stay on the surface of the water, but instead sinks to the bottom.

“If it is not promptly removed from the surface, it remains to wait until it is biodegraded by marine microorganisms,” said Natalia Gozak, the office director of Greenpeace Ukraine. “This can take decades.”

The lack of an immediate response means that large masses of contaminated sand need to be scooped out, essentially gutting the portions of the beaches around Anapa, according to Georgy Kavanosyan, an independent Russian environmentalist and hydrogeologist who arrived at the scene two days after the spill.

“The oil started sinking down into the sand in the initial days because there were not enough responders there,” Mr. Kavanosyan said.

Satellite images published by Mr. Kavanosyan showed two elongated stains near the tanker that ran aground, indicating new oil spills from it after two minor earthquakes in the area over the weekend.

“That vessel is a ticking bomb,” he said. “The most important thing right now is to pump out that oil and extract the ship.”

Officials reported that they had collected most of the oil from that spill by last Monday.

When Mr. Putin finally spoke out about the disaster, he described it as “one of the most serious environmental challenges we have faced in recent years.”

Mr. Putin ordered senior officials dispatched to oversee the efforts. A task force set up this month brought in several ministers to make plans for the cleanup and reconstruction, as well as for removing the tankers.

The oil spill’s longer-term effect on wildlife remains to be seen.

At least 58 dead dolphins have been found so far, the Delfa Dolphin Rescue and Research Center said in a statement on Saturday. The group sent a team out to the sea last Friday to reach the sunken tanker and confirm reports that the oil was still seeping out from it.

“Contamination was along the entire route,” it said. “Just five kilometers from the shore, common bottlenose dolphins and harbor porpoises were swimming in oil film and small fractions of fuel oil, to our great regret and alarm.”

At least 6,000 oil-smeared birds have been caught and cleaned by volunteers, but many are unlikely to survive, experts said. The spills are likely to kill tens of thousands of local birds, according to Greenpeace Ukraine.

Russian oil companies have increasingly turned to using dilapidated tankers that are not regulated or insured by Western companies.

President Volodymyr Zelensky of Ukraine and other Ukrainian officials have suggested the two 50-year-old tankers were part of Russia’s shadow fleet, which emerged after Western nation moved to punish Moscow economically over the invasion of Ukraine.

But Elisabeth Braw, a senior fellow at the Atlantic Council who has written several articles on the shadow fleet, said the ships were “rickety old tankers” that lacked several characteristics of shadow fleet vessels, which typically operate in the Baltic Sea and sail under the flags of other countries.

Both tankers involved in the spills are Russian-owned, and one of them had its license suspended and should not have been allowed to sail, according to the state-owned RIA Novosti news agency.

Questions were also raised about why the tankers, originally built for river navigation, were allowed at sea in winter storms in the first place.

Cassandra Vinograd contributed reporting from Kyiv, Ukraine.