Ukraine's Attorney General Ruslan Ryabushabka said on Tuesday that his office is reviewing cases about a company linked to former Biden's son Hunter Biden, after US President Donald Trump called on Ukraine and China to investigate the activities of his potential rival in the upcoming presidential election.

The prosecutor said in a press statement that his office is reviewing cases overseen by the Prosecutor's Office earlier, which was closed to decide on whether this is illegal, stressing that these files do not seem to mean the son of Joe Biden.

Hours earlier, the US president said China and Ukraine should open an investigation into Biden's son and son, adding that their actions in Ukraine and China were bad, he said.

Ukrainian authorities said earlier that the investigation of Borisma gas company focuses on the period before Hunter Biden joined the board of directors.

Hunter, the second son of the former US vice president, has become the focus of a political scandal in Washington that prompted Democrats to initiate a parliamentary trial against President Donald Trump over a telephone conversation with his Ukrainian counterpart Volodymyr Zelinsky in which Trump asked Kiev to investigate the activities of Joe Biden and his son in Ukraine.

A telephone call on July 25, released by the White House, revealed that Trump had pressed his Ukrainian counterpart to open an investigation into the activities of Joe Biden and his son.

The revelation came after a complaint from a secret US intelligence informant said Trump had withheld US aid from Kiev to pressure the Ukrainian president to heed his request to investigate the activities of Joe Biden and his son.

In 2014, while his father was Vice President Barack Obama and overseeing US relations with Ukraine, Hunter joined the board of directors of Borisma Holdings.

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Presidential elections
The developments come at a time when Joe Biden is one of the most likely Democratic candidates to win a ticket for his party to compete with Trump in the presidential elections scheduled for November 2020.

Biden said in remarks distributed by his campaign last Wednesday that he "will not allow Trump and those who help him to finance attacks on him to destroy him and his family." The Democratic candidate has vowed to defeat Trump hard.

President Trump ordered the dismissal of his country's ambassador to Ukraine, Mary Jovanovich, after complaints by Trump's attorney Rudy Giuliani and other allies of obstructing his efforts to persuade the Kiev authorities to investigate the biography of Joe Biden, The New York Times reported.

The dismissal of an ambassador
The State Department described Ambassador Jovanovic's departure as her job after her term expired, but information suggests she was called up three months before the end of the three-year diplomatic term.

According to those close to the article, State Department officials told her they could not protect her from attacks by the president and his allies.

The newspaper revealed that senior US diplomats had prepared the texts of letters addressed to the President of Ukraine in the hope of persuading him to open investigations on President Trump's political rivals.

The US ambassador to the European Union, Gordon Sundland, and the US special envoy to Ukraine, Kurt Volger, testified before committees in the House of Representatives as part of the ongoing investigation to start Trump's trial in hopes of isolating him.