KYIV, UKRAINE, MAY 16

Europe pushed to toughen its response Monday to Moscow's invasion of Ukraine, with Sweden joining Finland in deciding to seek NATO membership and European Union officials working to rescue proposed sanctions on Russian oil.

Ukrainian troops repulsed Russia's attempted advances and even rolled back the front lines in places. In recent days, Moscow's forces pulled back from around the northeastern city of Kharkiv after weeks of bombardment.

Ukrainian President Volodymyr Zelenskyy on Monday thanked his troops who pushed all the way to the Russian border in the Kharkiv region.

"I'm very grateful to you, on behalf of all Ukrainians, on my behalf and on behalf of my family," he said in a video message. "I'm very grateful to all the fighters like you."

Ukrainian border guards said they defeated a Russian attempt to send sabotage and reconnaissance troops into the Sumy region, some 90 miles (146 kilometers) northwest of Kharkiv.

And a glimmer of hope emerged for wounded Ukrainian troops trapped in the bombed remains of a giant steel plant, the last stronghold of resistance in the port city of Mariupol. The Russian Defense Ministry announced an agreement for the wounded to leave the steelworks for treatment in a town held by pro-Moscow separatists.

There was no immediate confirmation from the Ukrainian side, and there was no word on whether the wounded would be considered prisoners of war. Nor was it clear how many fighters might be evacuated.

As fighting raged in eastern Ukraine, the international response to Russia picked up pace. Sweden announced it will seek NATO membership, following a similar decision from its neighbor Finland. That would be a historic shift on the European continent for the two countries, nonaligned for generations.

Russian President Vladimir Putin, who launched the invasion Feb. 24 in what he said was an effort to check NATO's expansion but is now seeing that strategy backfire, warned that a military buildup on Finnish and Swedish territory "will of course give rise to our reaction in response."

NATO Secretary-General Jens Stoltenberg has said the membership process for both could be very quick, although member Turkey has expressed some reservations.

Swedish Prime Minister Magdalena Andersson said joining the 30-member military alliance is her country's best defense in the face of Russian behavior.

"Unfortunately, we have no reason to believe that the trend (of Russia's actions) will reverse in the foreseeable future," she said.

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.

But a proposed EU embargo faces opposition from a small group of countries dependent on Russian imports, including Hungary, the Czech Republic and Slovakia. Bulgaria also has reservations.

"We will do our best in order to deblock the situation," EU foreign policy chief Josep Borrell said. "I cannot ensure that it is going to happen because positions are quite strong."

Russia has been plagued by setbacks in the war, most glaringly in its failure early on to take Kyiv, the capital. Since then, much of the fighting has shifted to the Donbas, Ukraine's eastern industrial heartland, but that, too, has turned into a slog.

The two sides have been fighting village-by-village. Ukrainian forces have ground down the Russians but are taking losses, too.

"The chances, I think, of a rapid Russian success have gone," said Chris Tuck, a land warfare expert at King's College, London. "The Russian capacity for offensive operations is going to bleed away. ... I simply don't think that we're likely to see any major Russian breakthroughs."

The death toll, already many thousands, continues to mount.

In the Luhansk region of the Donbas, strikes overnight hit a hospital in Severodonetsk, killing two and wounding nine, including a child, the regional military command said. Overnight strikes also hit other towns. Regional military Gov. Serhiy Haidai said Ukrainian special forces blew up Russian-held railway bridges.