According to a US Department of Defense official, Russia last week increased its troops in eastern Ukraine from 30,000 to 40,000 men.

Austrian Chancellor Karl Nehammer expressed pessimism after a meeting with Russian President Vladimir Putin in Moscow.

The Russian army is preparing an offensive in eastern Ukraine, he said: "This battle will be fought with vehemence."

In the almost destroyed city of Mariupol, the Azov regiment of Ukraine reported an alleged attack by the Russians with a chemical substance.

There was no confirmation, but the United States and Great Britain reacted with concern.

Putin plans to meet with Belarusian President Alexander Lukashenko in the Far East on Tuesday.

Use of poison gas in Mariupol?

Shortly after a Russian threat to use chemical weapons in Mariupol, an unknown substance was dropped there with a drone, Azov announced late Monday evening.

However, the public Ukrainian TV broadcaster Suspilne reported that there was no confirmation from official bodies.

According to the Azov information, the people hit suffered from breathing difficulties and movement disorders.

Former Azov commander Andry Biletsky reported three people with symptoms of poisoning.

Western countries have warned Moscow of serious consequences if it uses chemical weapons or other weapons of mass destruction in the war that began nearly seven weeks ago.

Following the reports from Mariupol, British Foreign Secretary Liz Truss wrote on Twitter that any use of such weapons would be an escalation for which Russian President Putin and his leadership would be held accountable.

On Monday, the military spokesman for the pro-Russian separatists in Donetsk, Eduard Bassurin, said the Ukrainian fighters had been pushed into the Azovstal steel factory.

A fight for the fortifications on the factory premises would be too costly.

That's why you should rely on chemically armed troops, he said.

In the Syrian war, Russia itself did not use chemical weapons, but covered up and denied the proven dropping of bombs with poison gas by the Syrian government.

Selenskyj complains about the lack of weapons

According to President Volodymyr Zelenskyy, Ukraine lacks the heavy weapons to liberate the almost lost Mariupol.

"If we had airplanes and enough heavy armored vehicles and the necessary artillery, we could do it," he said in his nightly video address.

He is sure that Ukraine will eventually get the weapons it needs.

"But not only time is lost, but also the lives of Ukrainians." He also spoke of possible chemical weapons attacks by Russia.

This should be a reason for foreign states to respond even more harshly to Russian aggression, Zelenskyy said.

Because of the war, Ukraine is the most heavily mine-infested country in the world, said the President.

According to the regional administration, Russian units dropped so-called delay mines over the city of Kharkiv, which only react to movement.

The information could not initially be verified.

Artillery shelling killed at least eight civilians in the area.

Before the great battle in the east

The command of the Ukrainian army in eastern Ukraine announced that Russian attacks had been repelled in six places in the Donetsk region.

Ukraine maintains particularly strong troops there, which have held the front line against the separatist republics of Donetsk and Luhansk, which are controlled and equipped by Moscow, since 2014.

The Russian army is expected to try to "encircle and overwhelm" these Ukrainian units, said White House spokeswoman Jen Psaki.

The US government pointed out that Moscow had for the first time appointed a commander for the campaign in Ukraine, Army General Alexander Dvornikov.

He temporarily commanded the Russian operation in Syria, which used aerial bombardment to restore President Bashar al-Assad to power.

According to western estimates, a Russian attack could come from the north towards Kharkiv and Izyum.

Satellite images showed a mile-long convoy of infantry support vehicles, helicopter gunships and command posts off Izyum, a Pentagon official said.

A second pincer attack is expected from the south.

The upcoming battle reminded Ukrainian Foreign Minister Dmytro Kuleba of the tank battles in southern Russia during World War II.

During the advance in the wooded north of Ukraine after February 24, the Russian troops quickly got stuck, and the Ukrainians were able to immobilize many convoys from ambush.

In eastern Ukraine, Russian troops could be more compact and their supply lines are shorter, US military experts said.

In the open steppe landscape without cover, the armored Russian formations have the advantage.

Other experts predicted that supplies would remain a problem for the Russian army in the east as well.

That's going to be important today

On Russian Space Day, Putin is meeting with the Belarusian head of state Lukashenko at the Vostochny cosmodrome in the Far East.

The Russian army uses Belarus as a staging area against Ukraine and allegedly also launches airstrikes from there.

After the presidential meeting, Putin wants to take questions from journalists.

German President Frank-Walter Steinmeier visits eastern EU and NATO member Poland, which has taken in millions of refugees from Ukraine, on Tuesday.