President Vladimir Putin is ordering Russian forces to “maintain peace” in eastern Ukraine, the Kremlin has said.
The Russian leader has called on his defence ministry to dispatch "peacekeepers" to eastern Ukraine's two breakaway regions, according to a decree published early on Tuesday.
It is not immediately clear whether or when troops would enter the country.
Earlier today he signed the decree recognising the two regions - Donetsk and Luhansk - as independent statelets.
The move has been widely condemned as illegal, with western fears it could destroy long-running peace negotiations.
This evening Boris Johnson condemned the recognition as a breach of international law during a Downing Street press conference.
He said: "I gather just as I came into this press conference that Vladimir Putin has effectively announced that Russia is recognising the breakaway republics of Donetsk and Lugansk.
"This is plainly in breach of international law. It's a ... flagrant violation of the sovereignty and integrity of Ukraine.
"It is a repudiation of the Minsk process and the Minsk agreements, and I think it's a very ill omen and a very dark sign."
The UK government confirmed it will announce sanctions against Russia tomorrow shortly after.
Both the United States and the European Union have also responded by saying they would impose sanctions.
Increasing fears of a looming war come amid a spike in skirmishes in the eastern regions, which western powers believe Russia could use as a pretext for an attack.
Today Putin justified his decisions in a pre-recorded speech broadcast on Russia 24, in which he blamed Nato for the current crisis and called the US-led alliance an existential threat to Russia.
He went on to paint today's Ukraine as a modern construct that is inextricably linked to Russia.
Putin also charged that Ukraine had inherited Russia's historic lands and after the Soviet collapse was used by the west to contain Russia.
After the speech, he signed decrees in the Kremlin recognising Donetsk and Luhansk's independence and called on lawmakers to approve measures paving the way for military support.
It is a blow to Ukraine eight years after fighting erupted in the regions between Russia-backed separatists and Ukrainian forces.
Until now, Ukraine and the West have accused Russia of supporting the separatists, but Moscow has denied it, stating that Russians who fought there were volunteers.
Foreign Secretary Liz Truss said the UK will enact sanctions against Russia tomorrow.
Laws for the sanctions were already passed on February 10 as part of the government's measures "to urge the Kremlin to end its campaign of aggression in Ukraine".
Truss confirmed the plan in a tweet today, writing: "Tomorrow we will be announcing new sanctions on Russia in response to their breach of international law and attack on Ukraine's sovereignty and territorial integrity."
