(TNND) — Russia hit Ukraine with its largest aerial attack of the war, just about three weeks after President Donald Trump hosted Russian leader Vladimir Putin for face-to-face talks in the pursuit of peace.

“Whatever optimism there was is gone now,” Russian foreign policy expert Mark N. Katz said Monday.

Katz said Russia and Ukraine appear far from reaching a peace agreement.

“He's not slowing down, and if anything, he's hitting harder,” Katz said of Putin. “Now, the Russians do have this notion in their military doctrine of escalate to deescalate.”

It’s possible Putin wants to speed up an end to the war with his escalating attacks on Ukraine, said Katz, a professor emeritus of government and politics at George Mason University.

But Katz said it’s going to be harder than ever for the Ukrainians to trust Putin.

And Katz said Ukraine’s European allies are likely “nervous,” and Trump is probably “somewhat embarrassed” after the momentum toward a ceasefire stalled.

“Trump has called for negotiations between Putin and (Ukraine's President Volodymyr) Zelenskyy, whereas Putin has just set out terms. ‘This is what I want, and the war will end when Ukraine agrees.’ He's not interested in a negotiation or kind of give-and-take,” Katz said.

Trump met Aug. 15 with Putin in Alaska and then hosted Zelenskyy and top leaders from Europe at the White House a few days later.

Katz said Putin probably went to Alaska to feel out Trump’s appetite for pressuring Zelenskyy to end the war, with the minimum goal of delaying harsher sanctions.

Trump was asked about new sanctions on Sunday amid the fresh Russian attacks.

Trump indicated briefly to reporters that he was open to “the second phase of sanctions against Russia.”

And later Sunday, Trump expressed his displeasure with the new attacks.

“I'm not happy. I'm not happy about the whole situation,” Trump said, though he reiterated multiple times to reporters that he thinks the war in Ukraine will still get "settled."

Trump didn’t directly answer a reporter’s question about what the greatest obstacle to a peace deal was.

Trump said he plans to speak with Putin in the next few days.

Ukraine's Ministry of Defense said this weekend’s attack, which it called the largest air attack on Ukraine, involved over 800 weaponized drones and over a dozen missiles.

Ukrainian air defenders shot down close to 750 of the drones and four cruise missiles, the Ministry of Defense said.

But Zelenskyy said more than 40 Ukrainians were reported injured across the country, 20 of them in Kyiv.

At least four people were reportedly killed, and the Cabinet of Ministers building in the capital was damaged.

Zelenskyy called on his American and European allies to hit Russia with strong sanctions in the wake of the attack.

“Putin does not want negotiations, he is clearly hiding from them, so Russia’s fuel shortages and other economic troubles are the logical response to its refusal to agree to a ceasefire or a meeting at the leaders’ level,” Zelenskyy said in a video posted online.

Russia controls nearly a fifth of Ukraine, including Crimea and parts of Donbas that Russia had seized before its 2022 invasion.

Katz said Putin wants to keep Crimea, along with the new territory he’s captured and more that Ukraine still occupies.

“He wants them to leave and to surrender this territory to Russia,” Katz said. “And this would be especially bad for the Ukrainians, because this territory is heavily mined and (has) lots of defense works. So, if the Ukrainians gave that up, then they would be a lot more vulnerable because the territory they would fall back on hasn't been set up in this manner.”

Katz said Putin also wants a binding commitment that Ukraine will not join NATO.

He wants limits on Ukrainian forces, and he probably wants limits on outside military assistance for Ukraine, Katz said.

“And he's not willing to make any corresponding concessions,” Katz said.

At best, Putin seems to be willing to settle for a ceasefire in place for a couple of regions, as opposed to demanding that Ukraine give them up fully.

Ukrainians aren’t going to want to give up anything, Katz said.

“For them, the big concession is they agree to a ceasefire, not to regain lost territory,” Katz said.

But he said Ukraine won’t be willing to recognize captured territory as Russian.

And Ukraine won’t be willing to limit its armed forces.

Meanwhile, Katz said Trump just wants the war to end.

“(Trump) really does have an aversion to conflict. ... Doesn't think it's sensible. Bad for business,” Katz said.

Katz said Trump has probably been the leader with the most optimism that a peace deal could be reached.

But that optimism has been tested by Putin’s actions, including his participation in the "dictatorship-fest" in China with President Xi Jinping and North Korean leader Kim Jong Un, Katz said.

“I just think that these guys think that they're on the ascendant, (and) that America's on the decline,” Katz said of the Russian, Chinese and North Korean leaders. Indian Prime Minister Narendra Modi also met with Xi and Putin.

But will harsher sanctions work against Putin?

Katz said sanctions against Russia have reached diminishing marginal returns.

“The biggest thing that we could do is to cut off the Russian banks that accept payment for oil and gas exports,” he said. “We've cut off a lot of Russian banks, but not those. And so, if we cut off those, then we basically force everyone to – that's going to really hit Russia's revenue.”