How will Russia annex four regions that it has occupied, but only partially, when they are in the middle of a war zone?

Vladimir Putin has signed an accord to annex the occupied regions, after declaring that Russia would never give them up and would defend them with all means available.

Russia's president is on the back foot. His seven-month war is losing impetus while Ukraine's dramatic counter-offensive is derailing his original claim to the two eastern regions of Luhansk and Donetsk.

In all four regions under Russian occupation he staged so-called referendums, which were condemned by the international community as a sham and at times involved armed soldiers going from door to door to gather votes.

By annexing the eastern regions as well as Zaporizhzhia and Kherson in the south he will be able to send newly mobilised troops to a front line that Moscow says is more than 1,000km (620 miles) long. But he can also threaten the West if it continues to arm Ukraine with missiles used against what he has labelled Russian territory.

UN Secretary General António Guterres has condemned the annexations as a dangerous escalation: "Any decision to proceed... would have no legal value and deserves to be condemned. It cannot be reconciled with the international legal framework; it stands against everything the international community is meant to stand for; it flouts the purposes and principles of the United Nations."

It looks like a carbon copy of what President Putin did in March 2014, seizing the Crimea region from Ukraine, calling a referendum widely condemned by the international community and then annexing it anyway, through exactly the same constitutional process culminating in a vote in Russia's supportive parliament.

Except that it isn't. Crimea was seized with little bloodshed and came under total Russian control.

To varying degrees, all four of the regions now being annexed are still partially in Ukrainian hands. Together they make up 15% of Ukrainian sovereign territory.

The two eastern regions have been partly held by Russian-backed separatists since 2014, but after seven months of war only 60% of Donetsk can be claimed by Russia, and Luhansk is at the centre of a major Ukrainian offensive. Russian forces could be within hours of losing the strategically significant town of Lyman.

Zaporizhzhia's regional capital is very firmly Ukrainian-run, although in reach of Russian missiles, and Ukrainian forces are only a few miles from the city of Kherson.

How can you annex four regions you don't even control? Whatever the answer, Russia's leader has clearly been in a hurry to do so, announcing the self-styled referendums with barely any notice.

In his annexation speech, President Putin called on Ukraine to cease fire and return to talks, but he was clear there would be no return of occupied territory to Ukraine. Much of his speech was a diatribe against the West.

Kremlin spokesman Dmitry Peskov said on Friday that any strike on annexed territory would be regarded as an act of aggression.

That is not yet clear. Even Mr Peskov was unable to define where Russia would draw its new borders in occupied southern Ukraine. However, he said Russia would treat all of the Donetsk region as part of Russia. As for those parts not under occupation, he said those would have to be "liberated".

"It provides a diversion for the Russian people and the Russian state," says Paul Stronski of the Carnegie Endowment for International Peace, who believes little will change in practice.

Before the February invasion, Russia recognised the entirety of the two eastern regions as independent "people's republics" and now Moscow will define them as Russian territory. Vladimir Putin did the same with the two southern regions, ahead of his ceremony in the ornate St George's Hall in the Grand Kremlin Palace.

Having signed away Ukrainian sovereignty, he will submit the annexation treaties to the two houses of Russia's parliament and address it next week, before his 70th birthday next Friday. It can then be enshrined in Russia's constitution.

At this point Russia will enter into a new phase of existence, says exiled political commentator Ekaterina Shulman, becoming a "state with a delegitimised border". It will include fragments that are not just unrecognised by any other state or international organisation but also have no centralised administration, she argues.

President Volodymyr Zelensky has hit back at Russia's annexation moves by seeking accelerated membership of Nato.

That is a marked change from the start of the war, when he announced he would stop pushing for membership of the 30-strong Western defensive alliance because of Nato's concern about confrontation with Russia. He knows, however, that he will have to persuade every member state to agree, and Turkey for one is unlikely to.

He has also made clear that annexation will not bring the Kremlin what it hopes for: "Ukraine cannot and will not put up with any attempts by Russia to seize any part of our land."

Nobody really knows what is now in the Russian leader's mindset, but his anti-Western rhetoric has reached a new level. Clearly he wants the West to understand that Moscow views attacks on occupied regions of Ukraine as attacks on Russia itself.

But how big an escalation is this on the battlefield and beyond?

President Putin has already threatened to use every means at his disposal to protect Russian territory, including nuclear weapons. "This is not a bluff," he said. And his defence minister says Russia is fighting the West even more than Ukraine.

Ukraine's leader has dismissed the nuclear threat as a "constant narrative of Russian officials and propagandists". And Paul Stronski believes Russia's "destabilising rhetoric" is aimed at deterring the West, even though the West appears resolved to push back on it.

On Friday, Kremlin spokesman Dmitry Peskov said recent commentary on nuclear escalation was irresponsible: "We urge everyone to conduct themselves responsibly."