A former top adviser to President Donald Trump's election campaign has formally pleaded guilty to federal conspiracy and false statements charges in the special counsel's Russia investigation.

The plea by Rick Gates is a strong indication that he is planning to co-operate with special counsel Robert Mueller's investigation as it continues to probe the Trump campaign, Russian election interference and Gates' longtime business associate Paul Manafort.

The 45-year-old Gates made the plea at the federal courthouse in Washington.

It comes a day after Gates and Manafort were indicted in Virginia on new charges of tax evasion and bank fraud.

Manafort, who is Mr Trump's former campaign chairman, has said he is innocent.

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