Twelve Russian intelligence officers have been indicted on charges they hacked into Democratic email accounts during the 2016 US presidential election.

The US justice department also alleged the officers released stolen information in the months before Americans headed to the polls.

The indictment – which comes days before US president Donald Trump holds a summit with Russian leader Vladimir Putin – is the clearest allegation yet of Russian efforts to meddle in American politics.

US intelligence agencies have said the interference was aimed at helping the presidential campaign of Republican Donald Trump and harming the election bid of his Democratic opponent, Hillary Clinton.

The indictment lays out a sweeping and coordinated effort to break into key Democratic email accounts, including those belonging to the Democratic National Committee, the Clinton campaign and the Democratic Congressional Campaign Committee.

The Kremlin denied anew that it tried to sway the election.

“The Russian state has never interfered and has no intention of interfering in the US elections,” President Vladimir Putin’s foreign affairs adviser, Yuri Ushakov, said on Friday.

The charges come as special counsel Robert Mueller investigates potential co-ordination between Russia and the Trump campaign to influence the presidential election.

The indictment does not allege that Trump campaign associates were involved in the hacking efforts or that any American was knowingly in contact with Russian intelligence officers.

The indictment also does not allege that any vote tallies were altered by hacking.

Donald TrumpMr Trump is said to have been briefed over the developments (Jack Taylor/PA)

US deputy attorney General Rod Rosenstein said the internet “allows foreign adversaries to attack Americans in new and unexpected ways”.

He added: “Free and fair elections are hard-fought and contentious and there will always be adversaries who work to exacerbate domestic differences and try to confuse, divide and conquer us.”

Before Friday, 20 people and three companies had been charged in the Mueller investigation.

That includes four former Trump campaign and White House aides, three of whom have pleaded guilty to different crimes and agreed to co-operate, as well as 13 Russians accused of participating in a hidden but powerful social media campaign to sway American public opinion in the 2016 election.

Hours before the justice department announcement, Mr Trump complained anew that the special counsel’s investigation is complicating his efforts to forge a better working relationship with Russia. Trump and Putin are scheduled to hold talks Monday in Finland, a meeting largely sought by Trump.

He said at a news conference near London with Prime Minister Theresa May that he was not going into the meeting with Mr Putin with “high expectations”.