Russia and its ally Belarus launched a series of air-force exercises Monday along the border with Ukraine in an effort to boost cooperation ahead of what Ukrainian officials and military analysts believe could be a fresh effort by Moscow in the coming months to retake battlefield momentum.

Moscow has reorganized mobilization and training efforts, boosted arms production and last week appointed its top general in charge of Ukraine operations in an effort to increase efficiency on the battlefield and end a series of wins by Ukraine in recent months.

“The Kremlin is likely preparing to conduct a decisive strategic action in the next six months intended to regain the initiative,” said a note from the Institute for the Study of War, a U.S.-based think tank that publishes daily reports on the war in Ukraine.

Ukrainian authorities on Monday were also continuing rescue operations at an apartment building in eastern Ukraine where officials in Kyiv said a Russian missile struck over the weekend, killing at least 40 people. As the death toll has risen, the strike has turned out to be one of the largest attacks on civilians since the start of the war.

Russia used Belarus as a springboard for the initial phase of its invasion last year, but hasn’t so far fully enlisted Belarus as a battlefield ally in the first year of the conflict. Nonetheless, the show of strength between the two countries’ militaries underscores the will to maintain pressure on Ukraine.

The exercises involving joint air patrols and paratrooper landings begin the day after the Belarusian Security Council Chairman Pavel Muraveyko said Minsk was ready to respond to any threats from Ukraine and that the situation on the border was currently tense.  

Over the weekend, the two armed forces also carried out joint artillery exercises.

Russia remains committed to its maximalist demands on Kyiv, say U.S. and European officials, and its moves potentially to escalate its battlefield presence are taking place as Ukraine’s allies gear up for talks on Friday on whether to provide Kyiv with more sophisticated weapons to counter the Russian threat.

Representatives of more than 50 countries supporting Ukraine are set to gather in Ramstein, Germany, to discuss provisions for Kyiv and pledge fresh supplies later this month. The U.S.-led assembly, known as the contact group, includes all countries in the North Atlantic Treaty Organization and allies, including Japan, South Korea and Australia, offering lethal and nonlethal aid.

Ukraine is expected to receive Patriot missile systems, which Ukrainian officials say would hobble Russia’s missile attacks that have wreaked havoc on Ukraine’s civilian and critical infrastructure. Some Western officials also said that the first-ever shipment of Western-made main battle tanks could also be announced at the Friday meeting in Ramstein. 

Polish Prime Minister Mateusz Morawiecki told reporters Monday that Poland, which wants to send a company of Leopard 2 tanks to Ukraine but needs Berlin’s permission to re-export the German-made tanks, would soon be getting the necessary approval.

“I cannot imagine a situation where this consent would not be granted quickly,” he said, adding that he would speak to German leaders Monday. President Biden, he added, supported the provision of modern tanks to Ukraine.

German officials said they would send main battle tanks and approve third-country exports to Ukraine only as part of a joint NATO effort.

Kremlin spokesman Dmitry Peskov dismissed Western measures to support Ukraine, saying Monday they were unable to change the situation.

“These tanks will burn just like the others,” he said during a daily briefing with journalists. “The goals of the special military operation will be achieved,” he said, using the Kremlin’s euphemism for the invasion.

A still from a video released by the Russian Defense Ministry in December showed Russian troops taking part in drills at an unspecified location in Belarus.

Photo: /Associated Press

In a phone call Monday between Russian President Vladimir Putin and his Turkish counterpart Recep

Tayyip Erdogan who has become a mediator in the conflict, the two sides said that Russia and Ukraine would discuss another exchange of prisoners, primarily those who had been wounded.

Following talks between officials in Ankara, the human rights commissioners of Russia and Ukraine will now negotiate the exchange, the Kremlin said.

The two presidents also discussed cooperation in the energy sector, including turning Turkey into a regional hub for Russian gas.

The Turkish president’s office said the two leaders discussed efforts for Turkey to import Russian grain and mill it into flour in order to sell to African countries in need.

Mr. Erdogan and Mr. Putin have discussed the grain plan in the past, which comes amid Turkish efforts to address Russia’s demands to lower barriers to the export of Russian food and fertilizer products.

Last year, Mr. Erdogan helped broker a U.N.-backed agreement that allowed Ukraine to resume vital grain exports through key Black Sea ports following months of a Russian naval blockade. Mr. Putin threatened to abandon the deal citing what he said were obstacles to exporting Russia’s own grain. The Black Sea grain agreement helped lower global wheat and corn prices.

The two leaders also discussed Russian ammonia exports and Turkey’s willingness to “mediate lasting peace between Russia and Ukraine.” 

Ukrainian drones aimed at a Russian naval base in Crimea, on the Ukrainian peninsula occupied by Moscow, were intercepted by aerial defenses Monday morning, the local governor said in a statement.

Overnight, Russia launched a fresh volley of missile strikes, hitting the regional southern capital of Zaporizhzhia.

In Dnipro, the number of dead and wounded has steadily risen since Russian struck a nine-storey apartment block on Saturday with an Kh-22 cruise missile, an older type of missile that Ukrainian officials say are imprecise but can permeate Ukrainian air defenses. Ukraine’s emergency services said the figure had risen to 40 on Monday, however that number is expected to rise.

Mr. Peskov denied that Russian forces were attacking civilian targets, and alleged that the residential building had been hit by a stray air-defense missile. 

Separately, at least two people were killed and several wounded after Ukraine forces shelled on Monday the city of Donetsk in the eastern territory of Donbas occupied by Russia, according to local, pro-Russian officials. 

Ukrainian troops took part in an exercise near the country’s border with Belarus on Jan. 12.

Photo: GLEB GARANICH/REUTERS

Write to Thomas Grove at thomas.grove@wsj.com and Bojan Pancevski at bojan.pancevski@wsj.com