With a Kremlin decree, Vladimir Putin has imposed martial law in the four Ukrainian regions he claims to have annexed: Luhansk, Donetsk, Zaporizhzhia and Kherson regions.

It's not clear what difference, if any, that will make there: it certainly won't persuade Ukrainian troops to lay down their weapons. Kyiv is determined to win back lost territory.

But the Kremlin leader has also tightened security across Russia, with the introduction of three different security levels.

In those regions close to the border with Ukraine, such as Belgorod, Bryansk, Krasnodar and Rostov regions, as well as in annexed Crimea, a "medium level of response" has been declared. Measures include boosting security and public order: the decree also envisages restrictions on the movement of traffic, as well as on entry into and exit from these regions.

The next level down is "heightened readiness". This applies to central and southern regions of Russia, including Moscow. The presidential decree mentions "vehicle searches and traffic restrictions", as well as "tighter public order security".

In a message on social media, Moscow's Mayor Sergei Sobyanin tried to reassure Muscovites that "there will be no measures restricting the normal rhythm of life". That remains to be seen.

The lowest security level applies to the rest of the country (in effect, northern Russia, Siberia and the Russian Far East.)