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.