U.N. chief Antonio Guterres is "not surprised" after last week's leak of highly classified Pentagon documents reportedly indicated that the U.S. was spying on his phone, his spokesperson said Thursday.

"(Guterres is) not surprised by the fact that people are spying on him and listening in to his private conversations," U.N. spokesperson Stephane Dujarric said in a statement.

Dujarric criticized the leaks of highly classified U.S. intelligence documents that are believed to be the most serious security breach in a decade.

The far-reaching leak has shaken capitals from Washington to Kyiv to Seoul with revelations of the U.S. spying on allies and foes alike and the disclosure of sensitive military intelligence about the war in Ukraine.

Some of the most sensitive leaked details are purportedly related to Ukraine's military capabilities and shortcomings. They also contain information about U.S. allies, including Israel, South Korea and Türkiye.

Documents reportedly also showed that the U.S. was monitoring Guterres because it believed he was too soft on Russia.

According to the BBC, one document discussed how Guterres is too willing to accommodate Russian interests.

"The Secretary-General has been at his job for quite some time. He's been in politics and public figure for quite some time. So he's not surprised," Dujarric said in response to a question regarding the leaks.

What is surprising, he said, "is the malfeasance or incompetence that allows that such private conversations to be distorted and become public."

When asked if the U.N. chief will be protesting to the U.S., Dujarric responded: "We're not in the habit of revealing his private phone calls with various member states."

The leak also revealed a private conversation between Guterres and his deputy about the Black Sea grain deal.

The U.N. and Türkiye brokered the grain deal in July last year to help alleviate a global food crisis worsened by Russia's invasion of Ukraine disrupting exports from two of the world's leading grain suppliers.

Asked about the reports, Dujarric said: "The Secretary-General is not soft on any one country another."

"He has been very clear about the violations of international law, very clear about the violations of the charter. He says the same thing when he's in Moscow, when he's in Kyiv, when he's in New York, and that's in the open record," he said.

"Our efforts, his efforts have been to mitigate the impact of the war on the world's poorest. And that means doing what we can to drive down the price of food and the price of grain and fertilizer worldwide," he added.