Foreign analysts say Russia may be temporarily easing its offensive in eastern Ukraine as the Russian military attempts to reassemble its forces for what it hopes could prove decisive new assault on its neighbor. Russian President Vladimir Putin warned menacingly on Thursday that his forces “haven't even started” to fight.

Russian forces made no claimed or assessed territorial gains in Ukraine on Wednesday “for the first time in 133 days of war,” according to the Institute for the Study of War. The Washington-based think tank suggested Moscow may be taking an “operational pause,” but said that does not entail "the complete cessation of active hostilities."

“Russian forces will likely confine themselves to relatively small-scale offensive actions as they attempt to set conditions for more significant offensive operations" and rebuild the necessary combat power, the institute said.

Also read: 12 killed in Ukraine as Russia pounds rebel-claimed province

Russia’s Defense Ministry seemed to confirm that assessment, saying in a statement Thursday that Russian soldiers had been given time to rest.

“The units that performed combat missions ... are taking measures to recover their combat capabilities. The servicemen are given the opportunity to rest, receive letters and parcels from home,” read the statement, quoted by Russian state news agency Tass.

Putin warned Kyiv it should quickly accept Moscow’s terms to end the fighting or brace for the worst.

“Everybody should know that largely speaking, we haven’t even yet started anything in earnest,” he said in a menacing note.

Shelling continued in Ukraine's east, with the mayor of one Donetsk region city saying it was hit by the most powerful strike since the war began.

Kramatorsk Mayor Oleksandr Gorchenko said a Russian missile hit in the vicinity of the local government buildings shortly after noon. The city has been the center of the Donetsk regional government since 2014, when the city of Donetsk was taken over by Kremlin-backed separatists.

Earlier on Thursday, regional officials said at least one person was killed and six more were injured in the strike.

Volodymyr, 66, a resident who declined to provide his full name, sat in his overturned apartment, covered in blood.

“I was just sitting and drinking tea, and then there was an explosion," he told the Associated Press. "You can see the result of it.”

Asked if he felt safe staying in his apartment building block, he responded: “Is it safe right now anywhere in Ukraine? It all just happened in one moment, and that was it.”

Also read: High cost of Russian gains in Ukraine may limit new advance

In Kharkiv, Ukraine's second largest city, police said three people died and five were injured after the Nemyshlyanskii neighborhood was blasted from multiple-launch rocket systems.

“People suffered, for what?” local resident Sergey Pershin said as he watched medics tend to the wounded and put the dead in body bags.

“It's horrible. I'm so sick of it. Every night you wake up 10 times, you wait for it to end, wait until they start shooting. What are the bastards doing? There are residential buildings here.”

The Kharkiv region, which lies along the border with Russia, comes under daily shelling.

In Donetsk, 10 cities and villages came under shelling, and 35 buildings were destroyed, including a school, a vocational college and a hospital, officials said.

Donetsk is part of the Donbas, a mostly Russian-speaking industrial area where Ukraine’s most experienced soldiers are concentrated. Pro-Russian separatists have fought Ukrainian forces and controlled much of the Donbas for eight years. Putin recognized the independence of two self-proclaimed republics there just before Russia invaded Ukraine on Feb. 24.

Putin on Monday claimed victory in Luhansk, the other Donbas province, after Ukrainian forces withdrew from the last city they controlled there. The governor of Luhansk, Serhiy Haidai, denied Wednesday that the Russians had completely captured the province.

The Ukrainian military said Thursday that Russian forces also carried out shelling and helicopter strikes in the Sumy region in the northeast.

Even as the fighting continued, the British Defense Ministry said it thinks Russia's military is “reconstituting” its forces. A ministry intelligence assessment issued Thursday said the heavy shelling along the front line in Donetsk is likely intended to secure previous Russian gains.

Further hostilities were reported in the Black Sea. The Ukrainian military said Thursday a national flag again stood on a strategic island that Russian troops withdrew from last month.

Ukraine’s southern military command said Ukrainian military units had cleared Snake Island, an outpost off Ukraine’s southwestern coast vital for guaranteeing sea lanes out of Odesa, Ukraine's biggest port.

Ukrainian President Volodymyr Zelenskyy said the operation to take back the island took two months and was carried out by an array of military and intelligence forces.

“And now let every Russian captain, whether of a ship or a plane, see the Ukrainian flag on Snake Island and know that our state cannot be broken," he said in his nightly video address.

The Russian Defense Ministry said Thursday that a Russian military aircraft launched a missile strike on the island as Ukrainian forces attempted to plant the flag. "As a result, some of the Ukrainian military personnel were destroyed, the rest fled,” the ministry said. The claim could not be immediately verified.

When Russian troops withdrew from Snake Island on June 30, the Defense Ministry described it as a “a goodwill gesture” to ensure Ukrainian grain and other exports could resume.

Ukraine said Russia also fired two missiles at a Moldovan-flagged oil tanker in the Black Sea, setting it ablaze. Ukrainian officials said one hit the Millennial Spirit, which carried over 500 tons of diesel fuel, while the other went wide.

The ship has been without a crew, drifting at sea since the start of the war in February. Russia did not immediately acknowledge the strike on the vessel.

Ukraine's Foreign Ministry said it summoned the Turkish ambassador in Kyiv on Thursday over what it described as the theft of Ukrainian grain by a Russian ship.

The Zhibek Zholy was allowed to leave Turkey's Black Sea coast after Turkish authorities briefly detained it at Ukraine's request. Ukraine summoned the ambassador to complain.

Turkey, with its Bosporus Strait, is the key transit route for shipping out of the Black Sea. Ukraine has sought to pressure Ankara to stop Russian shipments of its grain, a vital source of revenue.

Also Thursday:

Zelenskyy said he spoke with Boris Johnson just hours after the British prime minister announced he would step down. Zelenskyy said he thanked Johnson, who visited Kyiv twice after the war began, for his support and especially for the weapons and money Britain has sent to Ukraine.

“And although this reflects the position of British society, the leadership and charisma of the country’s leader always have special meaning. ... It’s not surprising that Ukrainians feel a personal gratitude to Boris,” Zelenskyy said in his nightly address. He said Johnson and other British officials reassured him this strong support would continue.