A Ukrainian soldier atop a tank in Odessa on March 5, 2022. (Salwan Georges/The Washington Post)

WASHINGTON – Russian forces attempting to advance through Ukraine remain largely stalled as Ukrainians continue a strong defense of their country, destroying enemy equipment and inflicting an unexpected number of casualties after three weeks of fighting, a senior U.S. defense official said Wednesday.

Russia has lost about 10% of its forces used to invade Ukraine, though the U.S. has not seen “tangible proof” that the Russians has tried to send reinforcements so far, the official said.

“They are suffering losses every day: losses of people, losses of equipment, losses of aircraft,” the official said. “So, it certainly stands to reason that they would want to be exploring options to replenish those losses.”

As the war enters its fourth week, the official said predicting how long the war in Ukraine will continue is “a fool’s errand.”

“It's difficult to predict how long things are going to go,” the official said. “I think we've all been impressed with how the Ukrainians have fought back and slowed the Russian progress.”

However, Russian President Vladimir Putin still has “the vast majority” of the military power he spent months amassing along Ukraine’s borders with Belarus and Russia in preparation for an invasion, the official said. Since the attack started Feb. 24, Russia has launched more than 980 missiles at Ukraine.

“Let's not forget they still have an awful lot of combat-credible capability available for them inside Ukraine, and they're using it every day,” the official said.

So far, Russia has deployed about 75% of its fast-response units — known as battalion tactical groups — in Ukraine. Those are the units that Putin has “primarily relied upon” in the war, the official said.

Russia has nearly 170 BTGs that contain about 600 to 800 troops each kept “on constant alert” to deploy within an hour if called upon, Russian Defense Minister Sergei Shoigu told Russia’s state-owned TASS news agency in August.

Ukraine also has most of its military force available, which is putting up a “stiff resistance” against Russia, the official said.

President Joe Biden on Wednesday announced the U.S. will send Ukraine an additional $800 million in military aid in the coming weeks. The U.S. has now pledged $2 billion in security assistance to Ukraine since Biden took office in January 2021.

The Pentagon is now collecting the weapons and other equipment that Biden directed the U.S. to send to Ukraine, the defense official said. The aid package includes 100 tactical drones, 800 Stinger anti-aircraft systems, 9,000 portable anti-armor systems, various small arms, and body armor and helmets.

“We're sourcing these as fast as we can,” the official said. “The focus right now is on making sure that the things get to the Ukrainians as fast as possible because we understand the tyranny of time here."

As of Wednesday, Russian forces had “not made any significant advances” toward Kyiv and remain about 18 miles east of the capital city’s center, the official said. Ukraine also continues to control the nearby city of Brovary after days of Russian assaults.

The official said Russia has “mostly isolated” Chernihiv, which is about 88 miles north of Kyiv. However, Ukrainians there are working to “develop lines of communication to the south with some success.”

In southern Ukraine, where Russia has had more success, Russian troops have isolated Mariupol, the official said. Russian forces have been launching long-range missiles at the city for more than a week. They have destroyed civilian infrastructure, including a maternity hospital on March 9.

In southwest Ukraine, the Russian navy has used several surface ships in the Black Sea to shell some southern Ukrainian towns near Odessa, “but not in Odessa,” the official said. The city of nearly 1 million people is the third largest in Ukraine, behind Kyiv and Kharkiv.

Ukraine continues to defend and retain control of Odessa, as well as the nearby city of Mykolaiv, where Russian troops are stalled about six to nine miles outside the city’s center, the official said.

“[Russia is] running into a stiff resistance, no doubt about it,” the official said. “They continue to be frustrated and not making much progress on the ground.”