Russia has covertly spent more than $300m (£260m) since 2014 to influence politicians in more than 24 countries, the US has alleged.

The Department of State's allegation is based on a declassified US intelligence assessment released on Tuesday.

A senior official from President Joe Biden's administration said: "We think this is just the tip of the iceberg."

Russia has not publicly commented on the issue. Moscow itself has repeatedly accused the US of meddling abroad.

American intelligence "assesses that these are minimum figures and that Russia likely has transferred additional funds covertly in cases that have gone undetected", said the Biden administration official during a phone briefing.

The official was speaking on condition of anonymity.

The US intelligence assessment released in a cable did not name specific countries or officials believed to have been targeted by Russia - but said they spanned four continents.

The administration official said the US intelligence community was now privately briefing select countries on the alleged Russian covert financing. The briefings would remain confidential, the official added.

An administration source familiar with the findings alleged that Russia had spent about $500,000 to back Albania's centre-right Democratic Party in 2017 elections and also bankrolled parties or candidates in Bosnia, Montenegro and Madagascar, according to the AFP news agency.

The source also said the Kremlin had used Brussels as a hub for foundations and other fronts that back far-right candidates.

Fictitious companies were said to be used to fund European parties and to buy influence elsewhere.

The Russian authorities have so far made no public statements on the US claim.

Moscow has previously blamed America's CIA intelligence agency for interfering in other countries' affairs, including by backing various coups around the globe.

The US has meddled in foreign elections more than 80 times worldwide between 1946-2000, not including coups or attempts at regime change, according to a database kept by Carnegie Mellon University researcher Dov Levin.

On Tuesday, state department spokesman Ned Price called Russia's alleged covert funding an "assault on sovereignty".

Last year, US intelligence officials assessed in a report that Russian President Vladimir Putin was likely to have authorised attempts to influence America's 2020 election in favour of former President Donald Trump.

But it said no foreign government had compromised the final results.

Russia called the allegations of election interference "baseless".