Russia launched its largest drone attack on Ukraine overnight in the ongoing conflict that has lasted over three years, a Ukrainian official reported on Monday. President Donald Trump stated that Russian leader Vladimir Putin has become “crazy” for intensifying bombings on Ukraine at a time when the U.S. is attempting to mediate peace.

The assault on Sunday night consisted of 355 drones, according to Yuriy Ihnat, head of the Ukrainian air force’s communications department, speaking to The Associated Press.

The previous night, Russia fired 298 drones and 69 missiles of various types at Ukraine in what Ukrainians said was the largest combined aerial assault during the conflict. From Friday to Sunday, Russia launched around 900 drones at Ukraine, officials said.

The escalation appeared to thwart hopes that Trump’s peace efforts might lead to a breakthrough in the near term, as Putin looks determined to capture more Ukrainian territory and inflict more damage.

Russia has this month broken its record for aerial bombardments of Ukraine three times. The expansion of its air campaign came after Kyiv in March accepted an unconditional 30-day ceasefire proposed by the U.S. but Moscow effectively rejected it.

Russia is also still pushing along the roughly 1,000-kilometer (620-mile) front line, where it has made slow and costly progress, and is assembling its forces for a summer offensive, Ukraine and military analysts say.

“Only a sense of complete impunity can allow Russia to carry out such attacks and continually escalate their scale,” Ukraine President Volodymyr Zelenskyy wrote on Telegram on Monday. “There is no significant military logic to this, but there is considerable political meaning.”

He repeated his call for tighter international economic sanctions on Russia as a way of ending the war, because Russia’s “desire to fight must be deprived of resources.”

The European Union’s top diplomat, foreign policy chief Kaja Kallas, on Monday described the latest attacks on Kyiv as “totally appalling” and said the bloc intended to impose more sanctions on Russia.

Trump has threatened massive sanctions on Moscow, too, but so far hasn’t taken action.

Kremlin spokesman Dmitry Peskov said Putin makes decisions that are necessary to ensure Russia’s security and that the attacks were Moscow’s response to deep strikes by Ukraine.

Russia is grateful for American efforts to bring peace, which have included the countries’ first direct peace talks in three years, Peskov said.

He said negotiations are at “a decisive moment that is linked to emotional overloading for everyone and emotional reactions.”

The Ukrainian numbers for how many drones and missiles were fired could not be independently verified.

Trump made it clear he is losing patience with Putin. “I’ve always had a very good relationship with Vladimir Putin of Russia, but something has happened to him. He has gone absolutely CRAZY!” Trump wrote in a social media post on Sunday night.

Trump said Putin is “needlessly killing a lot of people,” pointing out that “missiles and drones are being shot into Cities in Ukraine, for no reason whatsoever.”

The U.S. president warned that if Putin wants to conquer all of Ukraine, it will “lead to the downfall of Russia!” But Trump expressed frustration with Ukrainian President Volodymyr Zelenskyy as well, saying that he is “doing his Country no favors by talking the way he does.”

Russia and Ukraine swapped hundreds more prisoners Sunday in the third and last part of a major exchange that was a rare moment of cooperation

Russia’s Defense Ministry said each side exchanged 303 soldiers, following the release of 307 combatants and civilians each on Saturday, and 390 on Friday – the biggest total swap of the war.

In their talks held in Istanbul earlier this month, Kyiv and Moscow agreed to swap 1,000 prisoners of war and civilian detainees each. The exchange has been the only tangible outcome from the talks.