A federal jury in the United States has charged 13 Russian nationals and three Russian companies as part of an investigation into alleged interference in the country's 2016 presidential elections.

The charges by the office of Special Counsel Robert Mueller described a conspiracy to sway public opinion and promote the campaign of Republican Donald Trump, against his Democratic rival Hillary Clinton.

"The indictment alleges that the Russian conspirators want to promote discord in the United States and undermine public confidence in democracy," Deputy Attorney General Rod J. Rosenstein said in a statement to reporters on Friday.

According to the indictment, the accused involved posed as Americans online, posted adverts against presidential Clinton and organised pro-Trump political rallies within the US.

Interference first began in 2014, according to the indictment, the year before Trump announced his candidacy for the presidency, which he eventually won.

"From in or around 2014 to the present, Defendants knowingly and intentionally conspired with each other (and with other persons known and unknown to the Grand Jury) to defraud the United States by impairing, obstructing and defeating the lawful functions of government," the indictment said.

The Russians specifically intended to interfere with US political and electoral processes, including the presidential election of 2016, it added.

Trump reaction

In a Twitter post later on Friday, Trump defended his campaign, saying it "did nothing wrong"

"Russia started their anti-US campaign in 2014, long before I announced that I would run for President. The results of the election were not impacted," he said.

Russia started their anti-US campaign in 2014, long before I announced that I would run for President. The results of the election were not impacted. The Trump campaign did nothing wrong - no collusion! — Donald J. Trump (@realDonaldTrump) February 16, 2018

Maria Zakharova, spokesperson for Russia's foreign ministry, called the charges "absurd".

"Thirteen people carried out interference in the US elections? Thirteen against the billion-dollar budgets of the special forces? Against the espionage and counter-espionage, against the newest developments and technologies? ... Absurd? Yes. But this is the modern American political reality," she wrote on Facebook.

In his press briefing, Rosenstein said the indictment contained "no allegation" that any American was a knowing participant in the meddling, adding that there was no judgment on whether the election outcome was affected.

Al Jazeera's Rosiland Jordan, reporting from Washington, DC, said the indictment detailed the 13 Russians and three companies made contact with at least two people working on Trump's campaign team.

"They pretended to be US persons, never telling these two unidentified campaign members, that they were in fact Russian citizens trying to be involved in the 2016 election," Jordan said.

"The indictment is clear that the two people contacted had no idea they were dealing with Russians not US persons."

In a separate White House statement, Trump called on people in the country to "unite as Americans to protect the integrity of our democracy and our elections."

"We cannot allow those seeking to sow confusion, discord, and rancor to be successful," Trump was quoted as saying.