KYIV, June 2 - 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, The Associated Press reported.

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 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% of the infrastructure and residential buildings have been destroyed by nonstop shelling, the mayor 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.

President Volodymyr Zelenskyy said in his nightly video address that there has been “some progress” in the battle for Sievierodonetsk but it was too early to give specifics.

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.

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.” (ap/ez)