Gen. Mark Milley said he tried calling his Russian counterpart after a missile hit Poland.
But he said he had "no success" contacting Gen. Valery Gerasimov.
The missile strike on the NATO member state sparked concerns that the Ukraine war could escalate.
Gen. Mark Milley, Chairman of the Joint Chiefs of Staff, said he unsuccessfully attempted to contact his Russian counterpart following a missile strike in Poland on Tuesday that killed two people.
The lack of communication between the senior military officials is concerning, given the risk of misunderstandings causing the Ukraine conflict to spiral into a larger war.
At a Pentagon press conference on Wednesday, Milley told reporters that his aides had "no success" when they tried to establish contact with Gen. Valery Gerasimov, chairman of the Russian General Staff, in the wake of the missile hitting Poland on Tuesday.
"My staff was unsuccessful in getting me linked up with General Gerasimov," Milley said.
In the aftermath of the strike, reports initially said the missile could have been fired by Russia, raising fears of direct military conflict between NATO and Russia.
Poland is a NATO state, and under Article 5 of the organization's founding treaty member states are pledged to come to the defense of other members if attacked.
However, Western officials now believe that it was more likely fired by Ukrainian air defense amid a barrage of Russian missile attacks on Ukrainian cities — a claim that Ukrainian President Volodymyr Zelenskyy has pushed back on.
The US and its NATO allies have supplied Ukraine with billions of dollars in aid and military equipment since Russia launched its invasion in February, and Russia has threatened the West with nuclear weapons over its support for the country.
Though official contacts between the US government and Kremlin have been limited, Milley and Gerasimov have had several private discussions since the war began. According to a Joint Chiefs of Staff readout from October, the two have discussed "several security-related issues of concern and agreed to keep the lines of communication open."
Open lines of communication "are vital if we are to avoid the risk of conflict caused by misconception, miscalculations or mistake," John Tierney, executive director of the Center for Arms Control and Non-Proliferation in Washington, told the Associated Press.
"It is unsettling to learn from General Milley that his counterpart was unreachable or not willing to engage when an explosion occurred in Poland," he added.
US defense officials told the AP that it is not unusual for Gerasimov to be unavailable for a call.
Milley also said during the press conference that he had spoken to his counterparts in Ukraine and Poland after the missile blast, and that an investigation to establish the source of the strike was ongoing.
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