KYIV, July 13 - Interior Minister Arsen Avakov, Ukraine’s most powerful minister who held the post for seven years, submitted a letter of resignation, his ministry said on Tuesday, without disclosing the reason for the move.

Neither the President Volodymyr Zelenskiy's office nor the ministry press service responded to Reuters requests for comment.

Avakov had run the ministry since 2014 but he and Zelenskiy had been at odds in recent weeks over the investigation into the killing of an investigative journalist in a car bombing in central Kyiv 2016.

“Bombshell: Ukraine's Interior Minister… filed a letter of resignation, but it has to be approved by the parliament,” Anders Aslund, senior fellow at the Atlantic Council, tweeted. “He is the most powerful minister, no reformer.”

Zelenskiy had said at a news conference in May he would talk to Avakov about whether he could remain in his post if a court decided that the suspects, veterans of the war with separatists in eastern Ukraine, were innocent.

In June, courts released two of the suspects from detention and placed them under house arrest pending their trial.

The court hearings have attracted protests, with activists saying the suspects were not involved in killing Pavel Sheremet. Protesters also demanded his resignation last June over allegations of police brutality.

Iryna Vereshchuk, deputy head of parliament's committee on security issues and a member of Zelenskiy's party, suggested the president had urged Avakov to step down.

"I think that they had an agreement that if the president asks him to write a letter of resignation, then Mr. Avakov will do it regardless of whether he wants it or not," Vereshchuk told reporters.

His resignation needs to be accepted by the parliament to take effect. Zelenskiy's party has a majority.

Lawmaker Oleksandr Kachura said on Telegram that a member of the party, Denys Monastyrskiy, had been nominated to succeed Avakov. (rt/tl/ez)