US President Donald Trump has attacked special prosecutor Robert Mueller, who is investigating Russian involvement in the US presidential election, accusing him of being biased toward Democrats.

Trump said in a tweet on Twitter that there was a conflict of interests supposed to prevent Muller from taking over the alleged complicity between the Russians and the current president's campaign when he was running for president in November 2016.

The US president called on the special investigator to disclose a working relationship between them described as bad, pointing out that he refused to be appointed head of the Federal Bureau of Investigation (FBI) the day before his appointment as a special prosecutor in the case of Russian intervention, and is a friend of the former Director of the FBI James Komi .

Trump Muller demanded an investigation with the Democrats, saying the real Russian complicity was in their favor.

He also accused him of appointing what he called angry Democrats in his team, and said some of them had worked for former presidential candidate Hillary Clinton, some of them in favor of former President Barack Obama, including Muller himself.

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Al-Jazeera correspondent Murad Hashim said it was the first time that the US president had made some clarifications about what he had previously said about a conflict of interest that should prevent Mueller from taking over investigations.

The New York Times has previously revealed that Trump was thinking of isolating Muller, and that there were differences between the two men regarding the drawings in one of the clubs in the cave, as well as the work of Mueller in the law firm was representing Jared Kouchner Trump's brother-in-law and his adviser.

It is noteworthy that Møller Republican was appointed under the former President George Bush as director of the FBI, and asked him after Obama took office two years in office.

Trump has repeatedly criticized Muller and his team, but this new attack comes as investigations have progressed and some members of the Trump campaign like Paul Manafort have already been charged, as well as accusations by the special investigator of Russian intelligence officers.

Muller and his aides questioned several of the suspects in the Russian intervention case, and it is not unlikely that Trump will call for an investigation.

Earlier, US attorney Rudolph Giuliani said the investigation into the Russian intervention would be completed in September.