The "battle of the Donbas" has begun, says Volodymyr Zelenskyy, pointing to Russia launching its anticipated attack on Ukraine's south-east region
After withdrawing its troops from the capital Kyiv, Russia is now focusing on the region of Ukraine which is close to its border.
It is an offensive that Ukraine has been expecting and its military has been preparing for, according to Mr Zelenskyy.
"We can now say that Russian forces have started the battle of the Donbas, for which they have long prepared," the President said.
"No matter how many Russian troops are driven there, we will fight.
"We will defend ourselves. We will do it every day."
In a social media post, Ukraine's military said the "second phase of the war has begun", with fighting feared to be more fierce than any since the war began on February 24.
Why is Russia focused on the Donbas region?
Donbas is in Ukraine's south-east and comprises the Donetsk and Luhansk regions.
It is the country's mostly Russian-speaking industrial heartland, and it is where Moscow-backed separatists have been fighting Ukrainian forces for the past eight years and have declared two independent republics that have been recognised by Russia.
In reality, the rebel separatists have taken control of about one-third of the region since the fighting began in 2014.
The 2014 fighting came during the Euromaidan, a turbulent period of protests prompted by then-Ukrainian president Viktor Yanukovych's refusal to sign a free trade agreement with the EU in favour of closer ties with Russia.
Ukraine's next government then excluded Russian-backed separatists from talks about the Donbas region, angering the Kremlin and the rebels.
Since then there has been tension in the region, with ongoing fighting between Ukraine and the separatist forces.
On February 24, the Kremlin launched what it called a "special military operation" in Donbas, but that has expanded into a full-scale invasion of Ukraine.
In recent weeks, after failing to take Kyiv, the Kremlin has declared that the capture of Donbas is its main goal in the war.
After withdrawing from the capital, Russian forces began regrouping and reinforcing ground troops in the east ahead of what is set to be a major battle.
Ukraine's Secretary of National Security, Oleksiy Danilov, said on Monday Russia had begun attacks on Donbas and other regions to the north.
"This morning, along almost the entire front line of the [eastern] Donetsk, Luhansk and Kharkiv regions, the occupiers attempted to break through our defences," Mr Danilov said.
How many troops does Russia have for the offensive?
It is difficult to know exactly how many Russian troops are in and around the Donbas region.
Initial estimates suggest the number could be as high as 60,000 soldiers.
A senior US defence official, speaking on condition of anonymity to discuss the Pentagon's assessments of the war, told The Associated Press there were now 76 Russian combat units, known as battalion tactical groups, in eastern and southern Ukraine.
This is an increase from last week, when there were 65 units.
That could translate to about 50,000 to 60,000 troops, based on the Pentagon's initial belief that a typical unit had 700 to 800 soldiers.
The US said Russia was also building up weapons and supplies as it attempted to encircle the region.
Pentagon spokesman John Kirby said Russia has added artillery, ground combat forces and other capabilities in recent days.
"We have seen the Russians continue to flow in enablers, capabilities that will help them fight in the Donbas going forward. That's artillery, rotary aviation helicopter support, command and control enablers," Mr Kirby said.
The capture of Mariupol, where Ukraine estimates 21,000 people have been killed, is seen as key because it would complete a land bridge between Russia and the Crimean Peninsula, seized from Ukraine in 2014.
The US defence official said if Russian forces succeeded in taking full control of Mariupol it could free up nearly a dozen battalion tactical groups for use elsewhere in Donbas.
How is Ukraine preparing to face the next wave of Russian attacks?
Ukraine's military has held firm so far, keeping Russia out of Kyiv and other major cities.
Doing the same in the east of the country, where it shares a border with Russia, will be a greater challenge because the Russian forces there are closer to their supplies.
Ukraine has called up military reservists aged between 18 and 60 while also stopping men the same age from leaving the country.
Thousands of Ukrainians have voluntarily signed up to fight against Russia's invasion.
And Ukraine's defence has also been heavily assisted by foreign support.
The United States recently announced a $US800 million ($1.1 billion) support package for Ukraine, as well as military equipment and training, as the battle in the east intensifies.
The United States is planning to teach Ukrainians how to use some of the weapons, including howitzers. Mr Kirby said this training would be performed outside Ukraine.
The United States has previously trained Ukrainian forces on how to use Switchblade drones.
Mr Kirby said half a dozen planes filled with military equipment had arrived in the region since US President Joe Biden made the announcement last week.
Mariupol continues to be one of the main concerns for Ukraine, with Russian attacks increasing.
Mr Kirby said Ukraine's resistance was holding firm, but it was under constant threat.
"The Ukrainians are still resisting," he said.
"The city has not fallen to the Russians, but they continue to pound it from the air and through long-range fire.
"We have seen indications in just the last few days that the Ukrainians not only have defended bravely, but they have been able to secure certain villages and towns in the Donbas."
ABC/wires
