In July, as he secretly readied an invasion of Russia, President Volodymyr Zelensky of Ukraine sent a very different signal in public: He wanted talks to end the war.

Speaking to the BBC, Mr. Zelensky said he had a plan to end the “hot stage” of the war this year. He dispatched his foreign minister on a surprise trip to China, a mission to improve Ukraine’s relationship with Russia’s most important partner. And he pushed for a series of international meetings, including one planned for Qatar in August, in which he hoped to rally backing for Ukraine’s positions and pave the way for a broader settlement.

His summer overtures departed from the two years of Mr. Zelensky’s refusing to offer any hint of concessions in the face of a Russian invasion that many Ukrainians believe aims to wipe their country off the map. And they made it all the more stunning when on Aug. 6, Ukrainian forces rolled into Russia’s Kursk region, delivering one of the most embarrassing moments for President Vladimir V. Putin in 30 months of war and confounding predictions that the two countries might be headed toward a cease-fire.

Kyiv is making a risky bet: that the incursion gives it new leverage for a favorable deal with the Kremlin, even as its military remains on the defense across much of the front line in Ukraine. Russians who know Mr. Putin expect him to lash out in response, believing that his military has the upper hand in personnel and weaponry.

There are already signs that cease-fire efforts suffered a setback. A diplomat involved in the talks said that Russian officials postponed a meeting planned to be held in Qatar this month to negotiate a deal in which both sides would stop attacks on each other’s energy infrastructure. The postponement was reported earlier by The Washington Post.

In comments reported on Monday by Russian state media, Mr. Putin’s foreign policy adviser, Yuri Ushakov, said, “At the current stage, given this escapade, we are not going to talk.” The length of any pause in negotiations, Mr. Ushakov added, “depends on the situation, including on the battlefield.”