KYIV, UKRAINE, APRIL 11

Russia claimed that it destroyed several air defense systems in Ukraine over the weekend, in what appeared to be a renewed push to gain air superiority and take out weapons Kyiv has described as crucial ahead of a broad new offensive in the east.

In one strike announced Monday, Moscow said that it hit four S-300 launchers provided by a European country it didn't name. Slovakia gave Ukraine just such a system last week but denied it had been destroyed. Russia previously reported two strikes on similar systems in other places.

Moscow's initial invasion stalled on several fronts as it met with stiff resistance from Ukrainian forces, who prevented the Russians from taking the capital and other cities. The failure to win full control of Ukraine's skies has hampered Moscow's ability to provide air cover for troops on the ground, limiting their advances and likely exposing them to greater losses.

With their offensive in many parts of the country thwarted, Russian forces have relied increasingly on bombarding cities. The war has flattened many urban areas, killed thousands of people and left Russia politically and economically isolated. Ukrainian authorities accuse Russian forces of committing atrocities, including a massacre in the town of Bucha, outside Kyiv, airstrikes on hospitals and a missile attack that killed at least 57 people at a train station.

In Bucha on Monday, the work of exhuming bodies from a mass grave in a churchyard resumed. Galyna Feoktistova waited for hours in the cold and rain in hopes of identifying her 50-year-old son, who was shot and killed more than a month ago, but eventually she went home for some warmth. "He's still there," her surviving son, Andriy, said.

Now, Russia is regrouping for a renewed push in the eastern Donbas region, where Moscow-backed separatists have been battling Ukrainian forces since 2014 and have declared independent states. Both sides are digging in for what could be a devastating war of attrition.

Russia has appointed a seasoned general to lead the effort, according to U.S. officials, though they do not see one man making a difference.

Ukrainian President Volodymyr Zelenskyy is meanwhile pleading for more Western aid, saying his forces need heavier firepower to resist the coming onslaught and push Russian forces back. Echoing his remarks in an AP interview, Zelenskyy said Sunday that the coming week could be crucial, with Western support to his country - or the lack thereof - proving decisive.

"To be honest, whether we will be able to (survive) depends on this," Zelenskyy said in a "60 Minutes" interview. "Unfortunately, I don't have the confidence that we will be receiving everything we need." In a video address to South Korean lawmakers on Monday, he specifically requested equipment that can shoot down Russian missiles.

But those armaments could increasingly come under attack as Russia looks to shift the balance in the 6-week-old war.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said the military used cruise missiles to destroy four launchers on the southern outskirts of the central city of Dnipro on Sunday. He said the military also hit such systems in the Mykolaiv and Kharkiv regions.

Lubica Janikova, spokeswoman for Slovakia's prime minister, denied Monday that the S-300 system it sent Ukraine had been destroyed. She said any other claim is not true.

It's not clear what that system included, but a senior U.S. defense official has said the Soviet-era system typically is comprised of four launchers. The official spoke on condition of anonymity to provide information in advance of the public announcement.

None of the Russian claims could be independently verified.

Ukraine has specifically asked for more S-300s in recent months, though it already had a number of the Soviet-built systems and other long-range air defense systems. It also has received batches of portable, shoulder-fired Western anti-aircraft weapons like Stingers, which are efficient against low-flying aircraft.

Austrian Chancellor Karl Nehammer was due to meet Monday in Moscow with Russian President Vladimir Putin, after meeting with Zelenskyy in Kyiv. Austria, a member of the European Union, is militarily neutral and not a member of NATO.

Questions remain about the ability of depleted and demoralized Russian forces to conquer much ground after their advance on Kyiv was repelled by determined Ukrainian defenders.

Britain's Defense Ministry said Monday that Ukraine has already beaten back several assaults by Russian forces in the eastern Donetsk and Luhansk regions - which make up the Donbas - resulting in the destruction of Russian tanks, vehicles and artillery.

In Washington, a senior U.S. official said Russia has appointed Gen. Alexander Dvornikov, one of its most experienced military chiefs, to oversee the invasion. The official was not authorized to be identified and spoke on condition of anonymity. Russia does not generally announce such appointments, and there was no comment from Moscow.

Dvornikov, 60, gained a reputation for brutality as head of Russian forces deployed to Syria in 2015 to back President Bashar Assad's government during the country's devastating civil war.

Until now, Russia has had no central war commander on the ground. But U.S. national security adviser Jake Sullivan, speaking Sunday on CNN's "State of the Union," played down the appointment's significance.

"What we have learned in the first several weeks of this war is that Ukraine will never be subjected to Russia," Sullivan said. "It doesn't matter which general President Putin tries to appoint."

Western military analysts say Russia's assault increasingly is focusing on eastern Ukraine - an arc stretching from Kharkiv, Ukraine's second-largest city, in the north, to Kherson in the south.

On Sunday, Russian forces shelled government-controlled Kharkiv and sent reinforcements toward Izyum to the southeast to try to break Ukraine's defenses, the Ukrainian military said. The Russians also kept up their siege of Mariupol, a key southern port in the Donbas that has been besieged since nearly the start of the war.

Oleh Synyehubov, the regional governor of Kharkiv, said Monday that Russians shelling had killed 11 people over the last 24 hours, including a 7-year-old child.

The Institute for the Study of War, an American think tank, predicted that Russian forces will "renew offensive operations in the coming days" from Izyum in the campaign to conquer the Donbas, which comprises Ukraine's industrial heartland.

But it said the outcome "remains very much in question."

In Mariupol, Russia deployed Chechen fighters, reputed to be particularly fierce. Capturing the city on the Sea of Azov would give Russia a land bridge to the Crimean Peninsula, which Russia seized from Ukraine and annexed eight years ago.

In a video posted on his Telegram channel, Chechen Leader Ramzan Kadyrov said Russian forces would launch a renewed offensive on Mariupol as well as Kyiv and other cities. "Our offensive work will be not only in Mariupol, but in all the other settlements, cities and villages," he said.

Mariupol's residents have lacked food, water and electricity since Russian forces surrounded the city. Hundreds of thousands have fled, though Russian attacks have also frustrated evacuation missions.

Vladislav Usovich, an 18-year-old conscript serving in Russia-backed separatist forces, advanced slowly with other fighters through residential areas around a factory in Mariupol on Sunday.

"I thought it would go better, I thought it would be faster. Everything is going slowly," he said. "The Ukrainians are prepared fighters. NATO trained them well."