Delegations from Russia and Ukraine arrived in Turkey on Monday for their second round of direct peace talks in just over two weeks, although expectations were low for any significant progress on ending the three-year war after a string of stunning attacks over the weekend.

Ukraine said Sunday it launched a surprise attack on four Russian airbases thousands of kilometres apart, destroying more than 40 warplanes. The raid was unprecedented in its scope and geographic reach, targeting bases in Russia's Arctic, Siberia and far east more than 7,000 kilometres (4,300 miles) from Ukraine.

The head of the Ukrainian security service, Vasyl Maliuk, who led the planning of the operation, said its success was "a major slap in the face for Russia's military power." He said the drones struck simultaneously in three time zones and the complex logistics took over a year and a half to prepare.

WATCH l Ukraine strikes deep into Russian territory in drones attack: 

Ukraine deploys swarms of drones to target Russian warplanes

On the eve of a second round of peace talks, Ukraine launched what it calls its longest-range attack against Russia, using drones to target Russian warplanes. Ukrainian authorities said they had been planning this for more than a year and a half.

Ukrainian President Volodymyr Zelenskyy called it a "brilliant operation."

Meanwhile, Russia on Sunday fired the biggest number of drones — 472 in all — at Ukraine since its full-scale invasion in February 2022, Ukraine's air force said, in an apparent effort to overwhelm air defences. This was part of a recently escalating campaign of strikes in civilian areas of Ukraine.

Low expectations

Amid the escalation in fighting, the talks in Istanbul appeared unlikely to make much progress. U.S.-led efforts to push the two sides into accepting a ceasefire have so far failed. Ukraine accepted that step, but the Kremlin effectively rejected it.

Senior officials in both countries have indicated the two sides remain far apart on the key conditions for stopping the war. The first round of talks, held on May 16, also in Istanbul, ended after less than two hours, though both sides agreed on the largest prisoner swap of the war so far, with hundreds of people freed from captivity on each side.

Zelenskyy said that "if the Istanbul meeting brings nothing, that clearly means strong new sanctions are urgently, urgently needed" against Russia.

The Institute for the Study of War, a Washington-based think-tank, said Sunday that "Russia is attempting to delay negotiations and prolong the war in order to make additional battlefield gains." The relentless fighting has frustrated U.S. President Donald Trump's goal of bringing about a quick end to the war.

A week ago, he expressed impatience with Russian President Vladimir Putin as Moscow pounded Kyiv and other Ukrainian cities with drones and missiles for a third straight night.

WATCH l Ukraine strikes deep into Russian territory in drones attack: 

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The official Russian response to the Sunday attacks was muted, with the attack getting little coverage on the state-controlled television. Russia-1 TV channel spent little over a minute on it Sunday evening, with a brief Ministry of Defence statement read out before images shifted to Russian drone strikes on Ukrainian positions.

Zelenskyy said the setbacks for the Kremlin would help force it to the negotiating table, even as it pursues a summer offensive on the battlefield.

"Russia must feel what its losses mean. That is what will push it toward diplomacy," he said at a summit Monday in Vilnius, Lithuania, with leaders from the Nordic nations and countries on NATO's eastern flank.

'Hard to underestimate' drones attack: analyst

Ukraine has occasionally struck airbases hosting Russia's nuclear capable strategic bombers since early in the war, prompting the Russian air force to redeploy most of them to the regions farther from the front line.

Because Sunday's drones were launched from trucks close to the bases targeted in five Russian regions, military defences had virtually no time to prepare for them.

Many Russian military bloggers chided the military for its failure to build protective shields for the bombers despite previous attacks, but the large size of the planes makes that a challenging task.

The attacks were "a big blow to Russian strategic air power" and exposed significant vulnerabilities in Moscow's military capabilities, according to Phillips O'Brien, a professor of strategic studies at the University of St. Andrews in Scotland.

"This is hard to underestimate," O'Brien wrote in an analysis. 

Deadly exchanges on Monday

Fierce fighting has continued along the roughly 1,000-kilometre front line.

Russian forces shelled Ukraine's southern Kherson region, killing three people and injuring 19 others, including two children, regional officials said Monday.

Also, a missile strike and shelling around the southern city of Zaporizhzhia killed five people and injured nine others, officials said.

Russian air defences downed 162 Ukrainian drones over eight Russian regions overnight, as well as over the annexed Ukrainian peninsula of Crimea, Russia's Defence Ministry said Monday. Ukrainian air defences damaged 52 out of 80 drones launched by Russia overnight, the Ukrainian air force said.

Vladimir Medinsky, an adviser to Putin, is leading the Russian delegation in Istanbul, with Defence Minister Rustem Umerov heading the Ukrainian delegation.

Officials said Turkish Foreign Minister Hakan Fidan would chair the talks, with officials from the Turkish intelligence agency also present.