BEIJING -- President Xi Jinping on Thursday sent condolences to Ukrainian President Volodymyr Zelensky and Iranian President Hassan Rouhani respectively over the crash of a Ukrainian passenger plane in Tehran.

In his messages, Xi said he was shocked to learn about the air crash, which killed all passengers and crew members on board.

On behalf of the Chinese government and people, and in his own name, Xi expressed deep sympathies over the victims and offered sincere condolences to the bereaved families.

The Ukrainian Airlines passenger plane, with 176 people on board, crashed Wednesday morning near Iran's Imam Khomeini International Airport shortly after taking off. All passengers and crew were killed, Iran's Press TV reported.

Ukraine International Airlines (UIA) representatives have said the crash, which claimed the lives of 167 passengers and nine crew members, was unlikely caused by crew error.

"We have information that the plane climbed 2,400 meters in altitude. The chances of the crew making a mistake are minimal; we simply don't think so. It's hard to say that something was wrong with the crew given their experience," Igor Sosnovsky, vice president for flight operations of UIA, told a news conference held at Boryspil International Airport.

UIA President Yevhenii Dykhne said that the Boeing 737-800 aircraft went through scheduled maintenance on Monday before departure. The plane was built in 2016 and delivered directly to the airline from the manufacturer.

Also on Wednesday, Iranian state television reported that both of the plane's black box voice and data recorders had been found.

Iran's civil aviation authority has said it will not hand over two black boxes recovered from the airliner to the plane's manufacturer Boeing or the United States, but will cooperate with Ukraine.

It is not clear to which country Iran will send the black boxes for analysis, Iran's Mehr News Agency quoted Ali Abedzadeh, head of Iran's Civil Aviation Organization, as saying, adding that Ukrainian is welcome to join the probe into the crash.

According to the New York Stock Exchange, shares in U.S. airplane giant Boeing Company declined 1.75 percent Wednesday after the crash.

Boeing shares finished at 331.370 points on Wednesday, down by 5.910 points or 1.75 percent from Tuesday, against the backdrop of rising U.S. stocks.

The company on Wednesday called the crash "a tragic event" and offered to "assist in any way needed," saying it is "in contact with our airline customer and (we) stand by them in this difficult time."

The International Civil Aviation Organization (ICAO) has pledged to assist the probe into the plane crash.

"ICAO is in contact with the states involved and will assist them if called upon. Its leadership is stressing the importance of avoiding speculation into the cause of the tragedy pending the outcomes of the investigation," the UN agency said in a statement released Wednesday.