This includes more indiscriminate attacks against civilian targets to inflict terror and break morale; expanding airstrikes across the breadth of the country; potentially hiring fighters from Syria and Iraq; and possibly conspiring for Belarus to join the conflict.

Western officials are also concerned that Vladimir Putin may even resort to using chemical weapons - banned under international law - in a "false flag" operation that he blames on Ukraine as another bid to justify his high-stakes, high-risk intervention.

Finally - though this list is not exhaustive - there is a danger that Moscow decides to target a western ally that is giving weapons to the Ukrainian military to bolster their defences.

Such a move could put Russia and the NATO defence alliance into direct military confrontation, which US President Joe Biden has said would mean World War Three.

Ukraine-Russia updates live - Putin 'not ready to end invasion'

More than two weeks into the invasion, a failure by Russia to roll tanks into the capital Kyiv and most other cities in the north, east and south of the country means its military is resorting to increasingly heavy bombardments against residential areas - even hospitals.

The ruthless move - which is killing and maiming children, women and men - is evidence of the Ukrainian military's successful defence operations despite Russia's superior firepower.

The willingness to inflict such indiscriminate harm also exposes the fallacy of President Putin's justification for his war - that he was acting to save Ukraine from what he falsely called the "neo-Nazi" government of Ukrainian leader Volodomyr Zelenskyy.

In another change, the initial invasion focused on the north, east and south of the country.

But early on Friday morning, Russian airstrikes hit military airfields in the west, which had until then been relatively untouched by the violence and seen as a safe haven.

A regional official said he believed the expansion in Russia's targeting marked a new step in the conflict, predicting that further Russian air attacks could follow.

There are also shifts among ground formations.

Elements of a 40-mile-long column of Russian military vehicles, that spent days barely moving to the north of Kyiv, have dispersed.

Defence Intelligence, the intelligence arm of the UK's Ministry of Defence, said this is "likely to support a Russian attempt to encircle the city".

Though the changed posture "could also be an attempt by Russia to reduce its vulnerability to Ukrainian counterattacks, which have taken a significant toll on Russian forces".

Multiple video clips have emerged of Ukrainian armed drones taking out Russian tanks and other equipment.

In fact, the seemingly stalled Russian advance across the country has forced top commanders to take greater risk themselves by moving closer to the front to try to regain momentum, western officials believe. This has led to at least three major generals being killed.

The tempo of Russian casualties could become unsustainable, which might be why President Putin is willing to recruit what his regime is referring to as "volunteers" - more than 16,000 of them - from countries like Syria and Iraq to join the war on Russia's side.

There have also been reports that Moscow is planning to deploy mercenaries from private military companies with suspected links to the Kremlin, such as the Wagner group.

In addition, Ukraine is concerned neighbouring Belarus, a key ally of the Kremlin, is ready to send its soldiers to bolster the Russian attack.

"As losses mount, Russia will be forced to draw on alternative sources to reinforce their overstretched regular forces," a Defence Intelligence update, posted on Twitter on Saturday, said.

Mr Putin has gambled his legacy on the war in Ukraine.

If it continues not to go his way, he may seek to escalate the violence even further by using chemical weapons such as sarin gas.

British and US officials have raised the alarm about this possibility, including Prime Minister Boris Johnson in an interview with Sky News's Beth Rigby.

The thinking goes that Moscow could launch a chemical weapons attack, which would add a new level of terror to the conflict, but then blame Ukraine and use that as a justification for even greater violence.

As Russia's offensive intensifies, so too is the level of military support that Western allies, including the UK, many European Union countries and the US and Canada, are giving to Ukraine's military, from ammunition to anti-aircraft missiles.

It means as well as having the will to fight to the death for their country - a level of commitment that Western officials believe is lacking within the Russian ranks - Ukrainian soldiers also increasingly have the firepower to be able to level the playing field slightly.

For the West, it has to perform a delicate balance act between providing sufficient support and not doing so much that it ends up in direct military confrontation with Russia.

Ukraine war: At what point should NATO confront Vladimir Putin?

This is why NATO allies have so far resisted increasingly urgent calls from Ukraine to impose a no-fly zone over their skies to stop Russian airstrikes.

However, Russia - again choosing escalation rather than de-escalation - on Saturday warned that it would treat Western arms shipments to Ukraine as legitimate military targets.

If that results in the Russian military attacking such a shipment on the territory of a NATO country then it would take the war to the alliance and that would mean a new era of conflict between two nuclear-armed foes.