Donald Trump’s effort to secure peace in Ukraine is reaching a decisive moment with Ukraine’s Volodymyr Zelenskyy challenging Vladimir Putin to engage in talks this week.

Following a weekend of hectic diplomacy, Zelenskyy said he will travel to Istanbul on May 15 where Putin has proposed direct negotiations between the two countries.

But the fragile process is surrounded by doubts and unresolved disputes — Zelenskyy and his European allies have insisted that Russia begins a 30-day ceasefire Monday and have threatened a dramatic increase in sanctions if Putin refuses. They say that the U.S. would join that effort although Trump himself has been more guarded in his public comments and Putin has ignored their demands.

“I will be in Turkey this Thursday,” Zelenskyy said late Sunday in his daily address to Ukrainians. “I hope that this time, Putin won’t be looking for excuses as to why he can’t make it. We are ready to talk, to end this war.”

The U.S. president injected fresh momentum into diplomatic efforts to end the war on Thursday with a call for Putin and Zelenskyy to agree to a ceasefire, raising the prospect of sanctions if Russian troops failed to observe it. On Sunday, he followed up, urging them to sit down and talk.

“HAVE THE MEETING, NOW!!!” Trump said in a post on Truth Social.

Trump reiterated his hopes at a news conference at the White House on Monday, calling the meeting “very important.”

“I was very insistent that that meeting take place,” Trump said. “I think good things can come out of that meeting.”

U.S. Senator Lindsey Graham, a Trump ally, said earlier this month that he has bipartisan support for a bill that would enact “bone-crushing” new sanctions on Russia including a 500% tariff on imports from countries that buy Russian oil, petroleum products, natural gas or uranium.

U.K. Foreign Secretary David Lammy is set to unveil fresh sanctions targeting Russia on Monday as Britain hosts a security meeting of European ministers for the first time.

The movement on Ukraine came amid a flurry of activity as Trump prepares to visit Gulf states this week. The U.S. has also announced progress in trade negotiations with China, nuclear talks with Iran, the war in Gaza and helped to ease tensions between India and Pakistan.

Trump has been coordinating more with his European allies and the Ukrainians in recent weeks as he seeks to pressure Putin to end a war that has stretched on for more than three years. Yet his public rhetoric hasn’t quite matched theirs, sowing doubts as to whether Trump is ready to start implementing a dramatic round of new sanctions.

German Chancellor Friedrich Merz, France’s Emmanuel Macron, Keir Starmer of the U.K. and Poland’s Donald Tusk visited Kyiv together on Saturday in a show of support for Zelenskyy and effectively gave Putin an ultimatum, warning that a new sanctions process will begin Monday unless Russia halts its attacks on Ukraine.

Trump had approved their statement and made clear in private conversations that he would go along with them — even with additional sanctions if necessary, according to a European government official familiar with the matter, who asked not to be named.

But asked whether Merz expects the U.S. president to enforce sanctions if Russia fails to observe the ceasefire, German chancellor’s spokesman Stefan Kornelius told reporters in Berlin Monday: “best check Trump’s communication channels.”

As the midnight Sunday deadline passed, there was no sign of Russia scaling back its attacks. Ukraine shot down 55 of 108 drones that Kremlin forces fired at targets across the country overnight, the army’s Air Defense said on Telegram early on Monday.

“Russians are completely ignoring the offer of a full and durable ceasefire starting on May 12,” Ukraine’s Foreign Minister Andrii Sybiha posted to X after calling into a meeting of his counterparts from Germany, Italy, Spain, Poland, France and the E.U.’s top diplomat in London on Monday. “They continue to attack Ukrainian positions all along the frontline.”

The ministers discussed coordination with the U.S. and next steps, including sanctions against Russian banking sector, the central bank and energy sector, which may be implemented together with a new military aid for Ukraine, Sybiha said on Telegram.

Macron spoke again with his European counterparts on Sunday afternoon and then held follow-up conversations with Zelenskyy and Trump to coordinate ahead of a crucial few days, according to a statement from the Elysee palace.

The European leaders are maintaining their stance that Russia must observe a ceasefire from Monday and should expect a strong response from Europe and the U.S. if attacks continue, one senior European official said Sunday, adding that the U.S. is fully supporting that position.

Even further sanctions from the European Union would be far from straightforward because they require unanimity among all 27 members and Hungary has repeatedly stalled previous efforts.

While Russia is open to dialogue on ending its war in Ukraine, it is “resistant to any kinds of pressure,” Kremlin spokesman Dmitry Peskov told CNN on Saturday.

Putin’s maximalist demands for ending the war have been starting to rankle Trump, who had promised and failed to deliver peace within the first 100 days of his second term in office.

The Russian president has insisted that any deal gives the Kremlin full control of four regions in the east and southeast of Ukraine that were annexed illegally in 2022 — even though they aren’t fully occupied by the Kremlin’s forces.

The U.S. has, instead, proposed freezing the conflict more or less along the current front lines, leaving most Russian-occupied territory in Moscow’s hands. The Trump administration is also prepared to recognize the Ukrainian region of Crimea that Putin annexed in 2014 as Russian, Bloomberg reported in April.

After Putin told reporters at the Kremlin over the weekend that he was ready for “serious talks,” Trump hailed “a potentially great day for Russia and Ukraine!” in a social media post.

Turkey had previously hosted talks between lower level Russian and Ukraine officials in 2022, just weeks after the full-scale invasion but those negotiations collapsed after the massacre of civilians by Kremlin forces was uncovered on the outskirts of Kyiv. Turkish President Recep Tayyip Erdogan also played a role in brokering a 2022 deal to allow the safe export of Ukrainian grain.

Erdogan, the prospective host of Thursday’s talks, spoke with Putin Sunday and said that he hoped negotiations would lead to a “permanent solution” to the war, according to a statement from his office.

Putin told reporters in the Kremlin that the talks should deal with the root causes of the conflict in Ukraine and establish a lasting peace. Moscow plans to enter the talks “without any preconditions,” he added.

“We do not rule out that during these negotiations it will be possible to agree on some new truces,” he said.

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—With assistance from Ania Nussbaum, Alberto Nardelli, Michael Nienaber and Alex Wickham.

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