Repeated attempts by the United States’ top defense and military leaders to speak with their Russian counterparts have been rejected by Moscow for the last month, leaving the world’s two largest nuclear powers in the dark about explanations for military movements and raising fears of a major miscalculation or battlefield accident.

Since Russia’s invasion of Ukraine, Defense Secretary Lloyd Austin and Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, have tried to set up phone calls with Defense Minister Sergei Shoigu and Gen. Valery Gerasimov but the Russians “have so far declined to engage,” said Pentagon spokesman John Kirby in a statement Wednesday.

The attempted calls by Austin and Milley, which have not previously been reported, come as Russia conducts operations near the borders of NATO members Poland and Romania while the United States and its European allies conduct air-policing operations over the Baltic Sea and pour weapons and equipment into Ukraine by ground transport.

Moscow and Washington maintain a deconfliction channel but current and former officials say contact from higher-ranking military leaders is needed to avoid unnecessary escalation or confusion.

“There is a high risk of escalation without the firebreak of direct contact between the most senior officials,” said James Stavridis, who served as the Supreme Allied Commander at NATO from 2009 to 2013. “Very young people are flying in jets, operating warships, and conducting combat operations in the Ukrainian war. They are not seasoned diplomats, and their actions in the heat of operations can be misunderstood.”

“We must avoid a scenario of NATO and Russia sleepwalking into war because senior leaders can’t pick up a phone and explain to each other what is happening,” he added.

Russia’s recent use of hypersonic missiles and other sophisticated weaponry against targets in western Ukraine have underscored the threat of spillover into a broader confrontation.

“The risks are obviously elevated currently,” said Rob Lee, a senior fellow at the Foreign Policy Research Institute. “Russia is striking targets in western Ukraine, which are not far from the border with NATO members, and the Ukrainian Air Force apparently continues to operate from that region, which means there is a risk that its aircraft could be mistaken for NATO aircraft across the border.”

U.S. defense officials have described the deconfliction phone line as a tactical mechanism to avoid miscalculations, especially when it comes to protecting NATO airspace or territory, but its functionality can be limited.

“It’s not set up to be a complaint line where you can just call in and just grouse about stuff,” said a U.S. defense official this week when asked about whether anything had been communicated through the channel. The official spoke on the condition of anonymity under ground rules set by the Pentagon.

Sam Charap, a senior political scientist at Rand Corporation, said calls by Austin and Milley serve a “fundamentally different purpose” from the deconfliction channel.

“One is about tactical accident avoidance. The other about strategic engagement,” he said. “It’s always important to maintain the strategic level to communicate our interests clearly and better understand theirs. When there’s no communication at that level, their worst-case assumptions, often based on poor information, are more likely to drive their behavior.”

As Russia’s battlefield setbacks become more pronounced and the conflict nears its second month, U.S. officials are concerned that Russian President Vladimir Putin may escalate militarily in the hopes of changing the trajectory of the war. As more dangerous weaponry and tactics are deployed, the risks of a wider conflict grow.

“A nightmare scenario would be a Russian missile or attack aircraft that destroys a U.S. command post across the Polish-Ukrainian border,” said Stavridis, a retired admiral. “A local commander might respond immediately, thinking the event was a precursor to a wider attack. This could lead to rapid and irreversible escalation, to include potential use of nuclear weapons.”

Stavridis said when he was Supreme Allied Commander, he could dial his Russian counterpart anytime “and did on several occasions to clarify a situation and de-escalate.”

The Pentagon holds the view that engagement between the U.S. and Russian defense leaders is “critically importance at this time,” Kirby said. Besides the deconfliction channel, the United States and Russia can also engage through the defense attache at the U.S. Embassy in Moscow or the relaying of messages to the Ministry of Defense.

Communications between the United States and Russia have been much more sparse since the war began last month. The U.S. ambassador to Russia, John J. Sullivan, has met with Russian officials most frequently, with on and off visits and calls in Moscow. President Joe Biden’s national security adviser, Jake Sullivan, spoke to his counterpart, Nikolay Patrushev, last week for the first time since the start of the conflict. Some U.S. and Russian military officials met last week at the Russian Ministry of Defense, CNN first reported.

Secretary of State Antony Blinken has not attempted any conversations with his counterpart, Russian Foreign Minister Sergei Lavrov, since the start of the conflict, according to U.S. officials.

It remains unclear why Russia’s top generals have refused to hold calls with their U.S. counterparts.

“I suspect that the problem lies with the Russian insistence that this is a ‘special military operation’ and unwillingness to admit the real nature of the war,” said Angela Stent, a Russia scholar at Georgetown University who served as a senior intelligence officer in the Bush administration.

The generals may also be waiting on Putin’s approval to make the calls, given the high stakes of the conflict, and he may not be signing off, Charap said.

Another theory is that Putin may now view the United States as a determined adversary bent on his downfall and not worth engaging. Russian officials bristled at Biden calling Putin a “war criminal,” saying it could lead to a complete break in relations.

Biden has sought to avoid a conflict by keeping U.S. troops out of Ukraine and U.S. aircraft out of its airspace.

“You’re talking about avoiding incidents with aircraft or at sea,” said Ben Hodges, a retired Army officer who served as commanding general of the U.S. Army Europe. “I’m sure they would’ve wanted to convey to Gerasimov and Shoigu that Russian pilots should not be launching missiles too close to the Polish border, but they would also want to talk about other places, not just Ukraine, where you have Russian aircraft.”

“I would also imagine they would want to convey - here’s what we’re doing, don’t interpret what we’re doing as a provocative act,” he added.

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The Washington Post’s Karen DeYoung contributed to this report.