Russian and Belarusian tennis players, who are banned from Wimbledon, will be allowed to compete in this year's U.S. Open, but only under a neutral flag, the U.S. Tennis Association said.

The USTA said it "previously condemned, and continues to condemn, the unprovoked and unjust invasion of Ukraine by Russia."

Russian player Daniil Medvedev, the current No. 1 player in the world, won last year's U.S. Open.

The only way to end the war in Ukraine, either on the battlefield or behind the negotiation table, is a parity of weapons, Mykhailo Podoliak, an adviser to the head of the Ukrainian Presidential Office, said on Monday.

“Being straightforward -- to end the war we need heavy weapons parity,” Podoliak said on Twitter.

According to the presidential adviser, Ukraine's military wish list includes 1,000 howitzers, 300 multiple launch rocket systems, 500 tanks, 2,000 armored vehicles and 1,000 drones.

“Negotiations are possible from a strong position, which requires parity of weapons,” Podoliak said. “There is simply no other way.”

Ukrainian Foreign Minister Dmytro Kuleba echoed Podoliak's plea for weapons on Monday in a tweet that recounted Ukraine's recent military triumphs achieved with limited resources.

“Ukraine has proven it can punch well above its weight and win important battles against all odds,” Kuleba said, pointing at victories in the battles of Kyiv, Chernihiv, Sumy and Kharkiv. “Imagine what Ukraine can do with sufficient tools,” the Foreign Minister added. Kuleba urged Ukraine's partners “to set a clear goal of Ukrainian victory and speed up deliveries of heavy weapons.”

Podoliak said a meeting of NATO defense ministers will be held in Brussels on June 15.

“We are waiting for a decision" on the weapons, Podoliak said.

The group, known as the Ukraine Defence Contact Group, will convene a meeting for the third time in a bid “to ensure that we're providing Ukraine what Ukraine needs right now,” U.S. Secretary of Defense Lloyd J. Austin III said at a press briefing in Bangkok, Thailand, on Monday.

Austin, who will be in attendance in Brussels, said that Ukraine needs support “in order to defend against Russia's unjustified and unprovoked assault.” The Secretary of Defense noted that looking ahead, Ukraine will require help “to build and sustain robust defenses so that it will be able to defend itself in the coming months and years.”

In his Monday evening address, Ukrainian President Volodymyr Zelenskyy called on Ukrainians to tell people in the occupied territories “that the Ukrainian army will definitely come.”

“Tell them about Ukraine. Tell them the truth. Say that there will be liberation,” the president said.

Meanwhile, Ukrainian officials played down threats of possible food shortages in the country due to the ongoing conflict. While Ukraine lost 25% of its sown area as a result of Russia' full-scale invasion, the country's food security was “in no way” threatened, Taras Vysotsky, the first deputy minister of Agrarian Policy, said at a press briefing for Ukrainian media on Monday.

“Despite the loss of 25% of sown areas, the structure of crops this year as a whole is more than sufficient to ensure consumption, which in turn also decreased due to mass displacement and external migration," Vysotsky said.

The deputy minister added that Ukraine has “already imported about 70% of essential fertilizers, 60% of plant protection products and about a third of the required amount of fuel” before the war erupted in late February. According to Vysotsky, current sowing volumes are enough to ensure domestic consumption and even exports.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Max Uzol and Yulia Drozd

Another mass grave has been dug up in Bucha, uncovering the bodies of seven men who authorities believe were tortured and killed during the bloody occupation of the city in March.

Police told ABC News their hands were tied with ropes behind their backs and they were shot in the knees and head.

"They were killed in a cruel way," police spokesperson Iryna Pryanyshnykova said. "These were civilian victims. The people here were killed by Russian soldiers and later they were just put into a grave to try to hide this war crime."

It's not clear why the men were killed, Pryanyshnykova said.

She said experts will analyze DNA to identify the victims.

-ABC News' Britt Clennett

Russian forces have pushed the Armed Forces of Ukraine out of the center of Severodonetsk, Ukrainian officials said.

"They are pressing in Severodonetsk, where very fierce fighting is going on -- literally for every meter," Ukrainian President Volodymyr Zelenskyy said in an address on Sunday evening.

Russian forces now control about 70% of the city, as intense shelling makes mass evacuation and the transportation of goods impossible, Sergiy Haidai, another Ukrainian official, said.

Around 500 people, including 40 children, are sheltering in the city's Azot chemical plant, Haidai said.

While the Ukrainians try to organize their evacuation, authorities of the self-declared Donetsk People's Republic have given an ultimatum to Ukrainian troops in the city.

"They have two options: either follow the example of their colleagues and give up, or die. They have no other option," said Eduard Basurin, deputy head of the People's Militia Department of the DPR.

-ABC News' Yulia Drozd and Tanya Stukalova

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan