Efforts were underway Tuesday to rescue the last of the defenders inside the Azovstal steel plant in the ruined port city of Mariupol after Ukrainian officials said the fighters had “completed their mission” and there was no way to free the plant by military means.

The Ukrainian military avoided using the term “surrender” to describe the effort to pull out of the steel plant to save as many lives as possible. Officials planned to keep trying to save an unknown number of fighters who stayed behind. It was unclear if soldiers evacuated to Russian-controlled areas would be considered prisoners of war.

The regiment that doggedly defended a steel mill as Ukraine’s last stronghold in the port city of Mariupol declared its mission complete Monday after more than 260 fighters, including some badly wounded, were evacuated.

Ukrainian President Volodymyr Zelenskyy said the evacuation to separatist-controlled territory was done to save the lives of the fighters who endured weeks of Russian assaults in the maze of underground passages below the hulking Azovstal steelworks. He said the “heavily wounded” were getting medical help.

“Ukraine needs Ukrainian heroes to be alive. It’s our principle,” he said. An unknown number of fighters stayed behind to await other rescue efforts.

The steel mill’s defenders got out as Moscow suffered another diplomatic setback in its war with Ukraine, with Sweden joining Finland in deciding to seek NATO membership. And Ukraine made a symbolic gain when its forces reportedly pushed Russian troops back to the Russian border in the Kharkiv region.

Still, Russian forces pounded targets in the industrial heartland of eastern Ukraine known as the Donbas, and the death toll, already many thousands, kept climbing with the war set to enter its 12th week on Wednesday.

Deputy Defense Minister Hanna Maliar said 53 seriously wounded fighters were taken from the Azovstal plant to a hospital in Novoazovsk, east of Mariupol. An additional 211 fighters were evacuated to Olenivka through a humanitarian corridor.

She said an exchange would be worked out for their return home. Officials also planned to keep trying to save the fighters who remained inside.

“The work to bring the guys home continues, and it requires delicacy and time,” Zelenskyy said.

Before Monday’s evacuations from the steelworks began, the Russian Defense Ministry announced an agreement for the wounded to leave the mill for treatment in a town held by pro-Moscow separatists. 

After nightfall Monday, several buses pulled away from the steel mill accompanied by Russian military vehicles. Maliar later confirmed that the evacuation had taken place.

“Thanks to the defenders of Mariupol, Ukraine gained critically important time to form reserves and regroup forces and receive help from partners,” she said. “And they fulfilled all their tasks. But it is impossible to unblock Azovstal by military means.”

The commander of the Azov Regiment, which led the defense of the plant, said in a prerecorded video message released Monday that the evacuation marked the end of the regiment’s mission.

“Absolutely safe plans and operations don’t exist during war,” Lt. Col. Denis Prokopenko said, adding that all risks were considered and part of the plan included saving “as many lives of personnel as possible.”

Elsewhere in the Donbas, the eastern city of Sievierdonetsk came under heavy shelling that killed at least 10 people, said Serhiy Haidai, the governor of the Luhansk region. In the Donetsk region, Gov. Pavlo Kyrylenko said on Facebook that nine civilians were killed in shelling.

The western Ukrainian city of Lviv was rocked by loud explosions early Tuesday. Witnesses counted at least eight blasts accompanied by distant booms, and the smell of burning was apparent some time later. An Associated Press team in Lviv, which was under an overnight curfew, said the sky west of the city was lit up by an orange glow.

But Ukrainian troops also advanced as Russian forces pulled back from around the northeastern city of Kharkiv in recent days. Zelenskyy thanked the soldiers who reportedly pushed them all the way to the Russian border in the Kharkiv region.

Video showed Ukrainian soldiers carrying a post that resembled a Ukrainian blue-and-yellow-striped border marker. Then they placed it on the ground while a dozen of the soldiers posed next to it, including one with belts of bullets draped over a shoulder.

“I’m very grateful to you, on behalf of all Ukrainians, on my behalf and on behalf of my family,” Zelenskyy said in a video message. “I’m very grateful to all the fighters like you.”

The Ukrainian border service said the video showing the soldiers was from the border “in the Kharkiv region,” but would not elaborate, citing security reasons. It was not immediately possible to verify the exact location.

Ukrainian border guards said they also stopped a Russian attempt to send sabotage and reconnaissance troops into the Sumy region, some 90 miles northwest of Kharkiv.

Russia has been plagued by setbacks in the war, most glaringly in its failure early on to take the capital of Kyiv. Much of the fighting has shifted to the Donbas but also has turned into a slog, with both sides fighting village-by-village.

Howitzers from the U.S. and other countries have helped Kyiv hold off or gain ground against Russia, a senior U.S. defense official said. The official, who spoke on condition of anonymity to discuss the U.S. military assessment, said Ukraine has pushed Russian forces to within a half-mile to 2.5 miles of Russia’s border but could not confirm if it was all the way to the frontier.

The official said Russian long-range strikes also appeared to target a Ukrainian military training center in Yavoriv, near the Polish border. There were no immediate reports of casualties.

Away from the battlefield, Sweden’s decision to seek NATO membership followed a similar decision by neighboring Finland in a historic shift for the counties, which were non-aligned for generations.

Swedish Prime Minister Magdalena Andersson said her country would be in a “vulnerable position” during the application period and urged her fellow citizens to brace themselves.

“Russia has said that it will take countermeasures if we join NATO,” she said. “We cannot rule out that Sweden will be exposed to, for instance, disinformation and attempts to intimidate and divide us.”

But President Recep Tayyip Erdogan of Turkey, a NATO member, ratcheted up his objection to their joining. He accused the countries of failing to take a “clear” stance against Kurdish militants and other groups that Ankara considers terrorists, and of imposing military sanctions on Turkey.

He said Swedish and Finnish officials who are expected in Turkey next week should not bother to come if they intend to try to convince Turkey of dropping its objection.

“How can we trust them?” Erdogan asked at a joint news conference with the visiting Algerian president.

All 30 current NATO members must agree to let the Nordic neighbors join.

Russian President Vladimir Putin said Moscow “does not have a problem” with Sweden or Finland as they apply for NATO membership, but that “the expansion of military infrastructure onto this territory will of course give rise to our reaction in response.”

Putin launched the invasion on Feb. 24 in what he said was an effort to check NATO’s expansion but has seen that strategy backfire. NATO Secretary-General Jens Stoltenberg has said the membership process for both could be quick.

Europe is also working to choke off funding for the Kremlin’s war by reducing the billions of dollars it spends on imports of Russian energy. A proposed EU embargo faces opposition from some countries dependent on Russian imports, including Hungary, the Czech Republic and Slovakia. Bulgaria also has reservations.