Initially it was only Russian outfits like Pravda and Interfax that were reporting it, but now Time mag’s man in Moscow says Ukrainian officials are hearing the same thing.

They may be Russian troops or they may be “Crimean irregulars.” The Interpreter is updating minute by minute on its liveblog. Whether this is a prelude to a larger Russian invasion or some isolated skirmish with Ukrainians at the base is unclear right now. The fear all along has been that Putin would engineer some sort of incident between the two sides and use it as a pretext to send in more troops, possibly on into eastern Ukraine too, but this would be an odd way to do it. A classic pretext would be to target ethnic Russian civilians in Crimea and then start landing Russian soldiers in the name of “protecting” them. Picking a fight with Ukrainian troops at a base obviates the “protection” excuse and makes it a more traditional war. Why would he do that?

No reports of shots fired yet, but sit tight. While this is going on at the base, there are also reports of 15 trucks’ worth of Russian paratroopers disembarking at Sevastopol.

Update: Russian forces demand R-S-P-E-C-T.

Update: Rather than get in a firefight and give Putin his pretext, win or lose, Ukrainian troops inside the base have retreated to a bunker.

Update: More hardball from Russia, in case their friends in the EU are seriously considering meaningful sanctions:

The Russians also sent menacing economic signals to the financially stressed interim central government in Kiev, which Russia has refused to recognize. Gazprom, the Russian natural gas monopoly, which supplies Ukraine with most of its gas, warned that it might shut off supplies unless Ukraine paid $1.89 billion that it owes the company.

“We cannot deliver gas for free,” Russia news agencies quoted Gazprom’s chief executive Alexei Miller as saying.

Gazprom cut off gas to Ukraine for nearly two weeks in January 2009, causing severe economic problems for Ukraine and for European customers elsewhere who were dependent on supplies delivered through Ukraine.

Update: In case you thought that Crimean independence referendum was a total sham orchestrated by Moscow and its loyalists to force the province away from Ukraine, rest assured that … you were totally right.

Voters in Ukraine’s Russian-occupied Crimea who vote in the March 16 referendum have two choices – join Russia immediately or declare independence and then join Russia.

So the choices are “yes, now” or “yes, later.”

Voting “no” is not an option.

Update: Here’s Time’s story on the attack on the base. Russian forces appear to be a mix of regulars and local pro-Russian paramilitary units.

Update: There’s a surrender in the works.