President Volodymyr Zelensky of Ukraine was pushing forward with diplomatic efforts on Saturday to reopen the Black Sea to Ukraine’s grain shipments, strategizing with NATO’s chief a day after discussing with the Turkish president the collapse of a deal that allowed ships to bypass Russia’s blockade.

Moscow pulled out of the yearlong agreement, a rare accord between Russia and Ukraine brokered by Turkey and the United Nations. Efforts to revive it have been plunged into doubt, as Russia has pummeled Ukrainian ports, striking grain stores and other infrastructure, and has vowed it will treat commercial ships in the Black Sea as potentially carrying military cargo.

“Due to Russia’s actions, the world is once again on the brink of a food crisis,” Mr. Zelensky wrote on Twitter late Friday. “A total of 400 million people in many countries of Africa and Asia are at risk of starvation. Together, we must avert a global food crisis.”

Turkey’s president, Recep Tayyip Erdogan, has been a key mediator between Russia and Ukraine since the full-scale invasion began last February, standing out from his NATO allies by keeping up friendly relations with President Vladimir V. Putin of Russia. Mr. Erdogan is expected to meet with Mr. Putin next month.

Mr. Zelensky said he had discussed prospects for peace with Mr. Erdogan and asked for help in returning prisoners of war, particularly members of the Crimean Tatar ethnic minority.

During the meeting, “President Erdogan stated that Turkey put forth an intense effort to make peace prevail,” the Turkish president’s office said on Twitter, adding that the call on Friday had taken place at Kyiv’s request.

On Saturday, Mr. Zelensky said that he and the NATO secretary general, Jens Stoltenberg, had discussed “the priority and future steps necessary for unblocking and sustainable operation of the Black Sea grain corridor.”

Russia has said that it would renew the deal, but only if other nations lift sanctions that they imposed in response to its invasion of Ukraine, a move that is unlikely. Moscow says that the deal has not been fair to Russia and that its producers have been forced to sell grain and other agricultural products at below-market prices.

On Friday, Mr. Erdogan told reporters that Russia wanted the grain corridor to remain, “but has some expectations from Western countries, and they need to take action.” He said he would discuss the issue with Mr. Putin on the phone and when they meet next month.

Moscow’s decision to end the deal came just days after the Turkish leader held a warm meeting with President Biden and said that Ukraine deserved “NATO membership with no doubt,” a move that potentially complicates relations with Mr. Putin, who has blamed NATO’s expansion, in part, for his decision to invade Ukraine, and raises questions about the possibility of reviving the deal.

Secretary of State Antony J. Blinken, speaking at the Aspen Security Forum in Colorado on Friday, accused Russia of “weaponizing food supplies” and said it would be “very, very difficult” for Ukraine to resume shipments of grain and other food products.

Tensions are also heightened in the region after an attack on Monday on the Kerch Strait Bridge, which links Russia to the occupied Crimean Peninsula, killed two civilians. Crimea was a key staging ground when Mr. Putin ordered the full-scale invasion of Ukraine and remains a key logistics hub for its war. Kyiv has made increasingly bold strikes on the peninsula, though it has neither confirmed nor denied responsibility for the bridge attack.

On Saturday, a drone attack on an ammunition depot on the peninsula prompted the authorities to evacuate a three-mile radius and briefly suspend some transportation, the Moscow-installed regional governor said Saturday.

The governor, Sergei Aksyonov, reported no damage or casualties. Video shared by Russian state media showed a thick cloud of smoke. The footage could not immediately be verified. Ukraine’s military confirmed a strike, saying it had destroyed an oil depot and warehouses.

In a video address to the Aspen Security Forum, an annual national security conference, Mr. Zelensky said on Friday that the Kerch Strait Bridge was a legitimate target for Ukraine and that it should be destroyed.

“The goal is to return the entire Crimea, because this is our sovereign territory,” he said. “The Kerch bridge is not some small logistical road. It is used to deliver ammunition and militarize the Crimean Peninsula.”

Mr. Zelensky also acknowledged that his country’s counteroffensive against dug-in Russian troops was advancing more slowly than expected because the operation got off to a late start.

“We did have plans to start it in the spring, but we didn’t, because, frankly, we had not enough munitions and armaments and not enough properly trained brigades. I mean, properly trained in these weapons,” Mr. Zelensky said.

The late start, he said, “provided Russia the time to mine all our land and build several lines of defense.”

Russia had many months to prepare for the counteroffensive, and the front is littered with mines, tank traps and dug-in troops, while Russian reconnaissance drones and attack helicopters fly overhead with increasing frequency.

Near the front line in southern Ukraine, Russia’s Defense Ministry said that a journalist for the RIA Novosti news agency was killed and three other journalists were wounded by shelling on Saturday. The ministry claimed that Ukrainian forces launched an attack using cluster munitions.

Russia provided no evidence of the claim and it could not be independently confirmed. Ukrainian officials did not immediately comment. U.S. officials said this week that Ukrainian forces had started using American-made cluster munitions against Russia’s defensive positions. For much of the war, Russia and Ukraine have used such munitions, which have been outlawed by many of America’s closest allies.

Eric Schmitt contributed reporting from Aspen, Colo.