"A fuel tank is on fire on one of the sections of the Crimean bridge," the agency said.
"The shipping arches are not damaged."
Crimean Railway (KZhD) reported that a fuel tank caught fire at the tail of a freight train.
"At 06.05, the equipment showed false employment on the railway tracks at the railway part of the transport crossing through the Kerch Strait. A fuel tank caught fire at the tail of the freight train. The locomotive with part of the cars was taken to the Kerch station. Emergency work is underway. The causes of the incident are being established," the message says.
Others were adamant that the collapse was indeed a well-planned "attack".
Mick Ryan AM, a leading Australian military strategist and Australian Army Major General, was among them.
"It is too early to ascertain the method of attack and the range of implications of this attack on the Kerch Bridge," he tweeted soon after.
"It is certainly a punch in the face for Putin on his birthday.
"Dropping a bridge span like this would take a lot of 'bang' [explosives] and good demolition design … The hardest bridges to drop are reinforced concrete like this. "
Despite Russian claims of a "car bomb", the Australian war expert disagreed.
"The amount of explosive required would be more than a few SF personnel could carry," Ryan said.
"A few trucks, or missiles or bombs would do the trick, if aimed at the right points of the bridge span.
"Ukrainians so far have been excellent at operational design and shaping operations in advance of their advances.
"This could be part of their design for taking back Crimea in the short term – or part of a deception operation to distract from other areas.
Ryan said if it was an attack, it was proof Russia could not protect annexed provinces.
AND
"This is a massive influence operation win for Ukraine," he said.
"Even if they didn't do it, it is a demonstration to Russians, and the rest of the world, that Russia's military cannot protect any of the provinces it recently annexed."
Whatever the cause, the collapse could prove to be a major headache for the Russians.
"It doesn't stop resupply to Crimea (there are boats and the route through Melitopol), but it makes holding Melitopol even more important for the Russians," Ryan said.
Stretching over 16km, Kerch is the longest bridge in Europe, constructed after Russia seized Crimea.
The bridge, which was built on the orders of Russian President Vladimir Putin, was a key transport link for carrying military equipment to Russian soldiers fighting in Ukraine.
