KYIV, Ukraine >> Ukrainian forces locked in a grinding battle for control of the country’s east struggled to hold off Russian troops and buy themselves some time Thursday while they await the arrival of the advanced rockets and anti-aircraft weapons promised by the West.

With the arms deliveries possibly weeks away, Ukraine is looking at a prolonged period of grueling combat, military analysts said.

“There’s a time lag, so the next few weeks are going to be pretty tough for our Ukrainian friends,” said retired U.S. Lt. Gen. Ben Hodges, former commanding general of U.S. Army forces in Europe.

Ukraine is intent on exhausting Russian forces, as evidenced by street-to-street fighting in the critical eastern city of Sievierodonetsk, Ukrainian military analyst Oleh Zhdanov said.

“And this can go on for quite some time,” he warned.

Britain on Thursday pledged to send sophisticated medium-range rocket systems to Ukraine, joining the U.S and Germany in equipping the country with some of the advanced weapons Kyiv had been begging for to shoot down aircraft and destroy artillery and supply lines.

Western arms have been critical to Ukraine’s success in stymieing Russia’s much larger and better-equipped military during the war, which was in its 99th day Thursday.

The Kremlin warned of “absolutely undesirable and rather unpleasant scenarios” if the latest Western-supplied weapons are fired into Russia.

“This pumping of Ukraine with weapons … will bring more suffering to Ukraine, which is merely a tool in the hands of those countries that supply it with weapons,” Kremlin spokesman Dmitry Peskov said.

Russian forces continued to pound towns and cities overnight and to tighten their grip on Sievierodonetsk in the eastern industrial Donbas region, which Moscow is intent on seizing. An estimated 800 people, including children, were holed up in bomb shelters at a chemical factory under attack in the city, the regional governor said.

In the neighboring city of Lysychansk, some 60 percent of the infrastructure and residential buildings have been destroyed by nonstop shelling, a local official said.

Britain’s Defense Ministry reported that Russia had captured most of Sievierodonetsk, one of two cities in Luhansk province that had remained under Ukrainian control. The Donbas is made up of Luhansk and Donetsk provinces.

Meanwhile, residents forced to flee the Kyiv area after Russian forces’ abortive attempt to storm the capital weeks ago confronted the overwhelming task of rebuilding their shattered lives.

Nila Zelinska and her husband, Eduard, returned for the first time to the charred ruins of their home outside Kyiv. They fled with her 82-year-old mother amid Russian shelling and airstrikes in the early days of the war.

A sobbing Zelinska recovered from the rubble a doll that belonged to one of her grandchildren, clutching it as if it were a real child.

“May there be peace on earth, peace so that our people are not suffering so much,” she said.

Speaking by video link to a security conference in Slovakia, Ukrainian President Volodymyr Zelenskyy called for even more weapons and sanctions against Russia to halt such horrors.

“As of today, the occupiers control almost 20% of our territory,” he said.

Zelenskyy said Russia had fired 15 cruise missiles in the past day and used a total of 2,478 missiles since invading. He said “most of them targeted civil infrastructure.”

British Defense Secretary Ben Wallace said Britain will send an unspecified number of M270 launchers, which can fire precision-guided rockets up to 80 kilometers (50 miles). Ukrainian troops will be trained in Britain to use the equipment, he said.

On Wednesday, the U.S. said it would supply advanced rocket launchers to Ukraine, and Germany agreed to provide up-to-date anti-aircraft missiles and radar systems.

Analysts think Russia is hoping to overrun the Donbas before any weapons that might turn the tide arrive. It will take at least three weeks to get the precision U.S. weapons and trained troops onto the battlefield, the Pentagon said.

Zhdanov, the Ukrainian analyst, said Russia stepped up missile strikes in response to the newly promised arms.

“Supplies of Western weapons are of great concern for the Kremlin, because even without sufficient weapons the Ukrainian army is daringly resisting the offensive,” he said.

Zhdanov predicted Russian forces will be exhausted when the fierce fighting in and around Sievierodonetsk is over, giving Ukraine time to get the weapons and prepare a counteroffensive.

Kyiv also got a diplomatic boost with the formal installation Thursday of a new U.S. ambassador to Ukraine, Bridget Brink.

Brink said her top priority “is to help Ukraine prevail against Russian aggression.”

“There is no place on the planet I would rather be,” she said after presenting her credentials to Zelenskyy. “President Biden has said that we’re going to be here, helping Ukraine, for as long as it takes. And that’s what we’ll do.”

Brink is Washington’s first ambassador in Kyiv since former U.S. President Donald Trump abruptly forced out Ambassador Marie Yovanovitch in 2019. She later became a key figure in the impeachment proceedings against Trump.

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Becatoros reported from Sloviansk, Ukraine. Yuras Karmanau in Lviv, Ukraine, Ricardo Mazalan in Kyiv, Oleksandr Stashevskyi in Potashnya, Ukraine, and Associated Press reporters around the world contributed to this report.