KYIV -- A senior Ukrainian security official submitted his resignation before President Volodymyr Zelenskiy began a visit to the United States this week, the president's office says.
"I confirm the information about my resignation," Oleksandr Danylyuk, the secretary of the National Security and Defense Council, wrote on Facebook on September 27.
Danylyuk, who was appointed after Zelenskiy took office in May, gave no reason for his decision.
He said he would stay in his post until the president had signed off on the decision.
The presidential office earlier said Danylyuk had sent his resignation letter before Zelenskiy traveled to the United States to attend the UN General Assembly in New York.
"The president will consider it after returning to Kyiv," a statement said.
Danylyuk’s surprise resignation may have been triggered by Zelenskiy's hesitation to break ties with a powerful oligarch, Ihor Kolomoyskiy, and take a clear position on the fate of the country's largest lender PrivatBank, AFP reported.
Danylyuk served as finance minister under Zelenskiy's predecessor, Petro Poroshenko, and oversaw the nationalization of the country's largest lender PrivatBank in 2016.
The announcement of his resignation also comes as a scandal involving a July phone call between Zelenskiy and U.S. President Donald Trump engulfs Washington.
During the call, Trump asked his Ukrainian counterpart to investigate his political rival Joe Biden, the former vice president and a possible Democratic candidate for the 2020 presidential election, and his son Hunter, who had worked for an energy company in Ukraine.
U.S. Democrats are arguing that Trump pressured Zelenskiy during the call to investigate the former vice president in order to help his own reelection bid.
Trump has denied any wrongdoing, and Zelenskiy said he did not feel "pushed” during the telephone call.