Even as Russia hammers eastern Ukraine with heavy artillery, it is cementing its grip on the south, claiming to have restored roads, rails and a critical freshwater canal that could help it claim permanent dominion over the region.

The extension of Russian infrastructure into the occupied south could allow Moscow to fortify a “land bridge” between Russia and Crimea and build on efforts to claim control through the introduction of Russian currency and the appointment of proxy officials.

Russia’s defense minister, Sergei K. Shoigu, said on Tuesday that the military, working with Russian Railways, had repaired about 750 miles of track in southeastern Ukraine and set the conditions for traffic to flow from Russia through Ukraine’s eastern Donbas region to occupied territory in Kherson and Crimea.

Mr. Shoigu also said that water was once again flowing to Crimea through the North Crimean Canal — an important source of freshwater that Ukraine cut off in 2014 after the Kremlin annexed the peninsula. Mr. Shoigu claimed that car traffic was now open between “continental” Russia and Crimea.

Mr. Shoigu’s claims of restored roads and rails could not be immediately verified.

Satellite imagery reviewed by The New York Times showed that water was flowing through the parts of the canal in Crimea that were dry until March. Russian engineers blew open a blockage in the canal in late February, days after Russian forces invaded Ukraine. Ukrainian officials did not immediately comment on Wednesday.

The North Crimean Canal, a 250-mile-long engineering marvel built under the Soviet Union, had channeled water from Ukraine’s Dnipro River to the arid Crimean Peninsula until President Vladimir V. Putin seized it in 2014.

After Crimea’s annexation, Ukraine dropped bags of sand and clay into the canal to prevent the Russian occupiers from benefiting from the valuable freshwater.

Instead of flowing to Crimea, the canal was used to irrigate the melon fields and peach orchards in Ukraine’s Kherson region to the north.

Ukrainian officials said that cutting off the water was one of the few levers at their disposal to inflict pain on Russia without using military force.

For the Kremlin, the blockage represented a vexing and expensive infrastructure challenge, with Crimea’s residents suffering chronic water shortages and occasional shut-offs at the tap.

When Mr. Putin massed troops on Ukraine’s border last year, some analysts speculated that the canal was one of the prizes the Kremlin wanted.

Even as Russia sought to entrench its control in the south this week, a clandestine battle has emerged inside the occupied regions, involving Kremlin loyalists, occupying Russian forces, Ukrainian partisans and the Ukrainian military.

On Tuesday, Ukrainian media posted video of what they said was an explosion at a cafe in the occupied city of Kherson that had served as a gathering place for people collaborating with Russian forces. Russian state media described it as an act of “terror.”

It was the latest in a series of attacks targeting Russian supporters and proxies. It came amid reports — most impossible to independently verify — of Ukrainian guerrillas blowing up bridges, targeting rail lines used by Russian forces and killing Russian soldiers on patrol.

Oleksiy Arestovych, an adviser to the Ukrainian president, said that there was a focused guerrilla movement operating in the south. “Partisans are fighting very actively,” he said on his YouTube channel.

In the east, where both armies are fighting for control, Ukrainian officials were weighing whether to withdraw their forces in the city of Sievierodonetsk, the last major pocket of Ukrainian resistance in the Luhansk region.

Sievierodonetsk has been blasted by weeks of Russian shelling, and President Volodymyr Zelensky of Ukraine referred to the city and its neighbor, Lysychansk, on Monday as “dead cities,” physically destroyed and nearly empty of civilians.

“Fighting is still raging and no one is going to give up the city, even if our military has to step back to stronger positions,” Serhiy Haidai, the Ukrainian military governor of the Luhansk region, said on Ukrainian television, according to Reuters.

Moscow’s announcement that it was extending its ties to the occupied south seemed certain to be greeted in Ukraine as further evidence of Russia’s determination to break Ukraine apart and pillage its natural resources.

“Russia is trying to build infrastructure for military supply,” said Mykhailo Samus, deputy director for international affairs at the Center for Army Studies, Conversion and Disarmament, a research group in Kyiv, the Ukrainian capital.

“Maybe they try to steal the agriculture, food products from occupied territories,” he added.

The Russian authorities said that the first train had traveled from the occupied city of Melitopol to Crimea carrying grain — freight that Ukrainian officials say was stolen from Ukrainian farmers forced to hand over their crops for a pittance or nothing at all.

Russia has blockaded Ukraine’s Black Sea ports since the start of the war, trapping more than 20 million tons of grain meant for export and deepening a global food crisis. Dimming the long-term outlook, grain silos in Ukraine are still about half full, the Ukraine Grain Association said on Wednesday, raising the possibility that much of this year’s crop could be left in the fields.

On Wednesday, the Russian and Turkish foreign ministers held talks focused on allowing Ukraine’s grain to reach global markets through the Black Sea.

But the Russian foreign minister, Sergey V. Lavrov, minimized the issue, suggesting that a global food catastrophe caused by a Russian blockade was a Western exaggeration.

“The current situation has nothing to do with the food crisis,” Mr. Lavrov said at a news conference in Ankara, the Turkish capital. “The Russian Federation is not creating any obstacles for the passage of ships and vessels.”

He blamed Ukraine, saying that its naval mines and refusal to use humanitarian corridors offered by Russia in Black Sea shipping lanes were stalling exports.

The Turkish foreign minister, Mevlut Cavusoglu, disagreed, saying that there was a global problem, but that it involved both Russian and Ukrainian products.

“The food crisis in the world is a real crisis,” Mr. Cavusoglu said, noting that Russia and Ukraine together supply about one-third of the world’s grain products.

Mr. Cavusoglu said that a mechanism was needed to get not just agricultural products from Ukraine out through the Black Sea, but also Russian fertilizer, which is vital for global agriculture.

He suggested that the answer lay in a United Nations proposal that the international community provide guarantees for the shipments that addressed security concerns on both sides.

Ukraine was not invited to the talks in Ankara, and its government and Russia’s each blame the other for the lack of exports.

The two countries normally supply about 40 percent of wheat needs in Africa, according to the United Nations.

Ukrainian officials are deeply skeptical of a promise by Mr. Putin, which Mr. Lavrov repeated, that if harbors were demined, Russia would not exploit them to dispatch an invasion fleet. Russian warships have also been patrolling Black Sea shipping lanes.

Oleksii Danilov, the secretary of Ukraine’s National Security and Defense Council, said on Twitter on Wednesday, “Our position on the supply of grain is clear: security first.” He accused Russia of “artificially creating obstacles to seize the market and blackmail Europe over food shortages.”

The United States has cited satellite imagery of cargo ships to accuse Russia of looting Ukrainian wheat stocks that it exported, mostly to Africa, echoing Ukrainian government allegations that Russia has stolen up to 500,000 tons of wheat, worth $100 million, since it invaded Ukraine in February.

Wheat is not the only Ukrainian resource prompting alarm. As Ukraine braces for what promises to be a difficult winter, Mr. Zelensky said that the country would not sell its gas or coal abroad. “All domestic production will be directed to the internal needs of our citizens,” he said.

Reporting was contributed by Valerie Hopkins, Ivan Nechepurenko, Malachy Browne, Neil MacFarquhar, Safak Timur and Anushka Patil.