Image copyright Getty Images Image caption The source of the smoke from Russia's consulate in San Francisco was the subject of speculation

Russian diplomats at its closed consulate in San Francisco have been formally accused of violating state laws following a fire early this month.

They might have to pay a fine over the offence, officials say.

It is not clear what materials were burned before the Russians vacated the premises in San Francisco - and annexes in New York and Washington DC.

The move follows Russia's expulsion of US diplomatic staff amid a row over alleged Russian election interference.

Thick black smoke was seen billowing from a chimney of the San Francisco consulate on 1 September.

Firefighters said "unknown items" were burned at the consulate fire, which Russia said the fire was part of "efforts to conserve the building".

Air quality officials in San Francisco say that diplomats have been issued with a Notice of Violation (NoV) for a rubbish burning incident. Burning rubbish is illegal in the city.

An NoV is a formal record of a violation of state law, with offenders liable to penalty fees - or in more serious cases - civil or criminal prosecution.

In most cases, a NoV can be settled by taking corrective action and paying the penalty fee.

The row - over the annexation of Crimea from Ukraine and allegations of interference by Russia in the US presidential elections - escalated in December when President Barack Obama ordered the expulsion of 35 Russian diplomats.

Russia denies allegations of interfering to favour the election of Donald Trump as US president.

President Vladimir Putin announced on 31 July a reduction of 755 US diplomatic staff in Russia, in retaliation for the US sanctions.

Russia was told at the end of August that it must close its San Francisco consulate - and annexes in New York and Washington - in response to its "unwarranted" order to the US to reduce the number of its diplomatic staff in Russia.