The Pentagon on Friday announced a new aid package worth up to $400 million for Ukraine, including four additional High Mobility Artillery Rocket Systems (Himars).
The latest package approved by President Joe Biden brings the total US military assistance to Ukraine to $7.3 billion since Russia invaded on February 24.
It also brings the number of Himars provided to Ukraine to 12, with eight already deployed in the country, a senior US defence official said.
The Himars is a mobile unit that can simultaneously launch several precision-guided missiles. Russia also operates multiple-rocket launchers, but the Himars has a superior range and precision, with its rockets able to fly as far as 80 kilometres.
The US has trained more than 100 Ukrainian troops on operating Himars, the Pentagon said.
The US official said there was “no truth” to Russia’s claims that its forces destroyed two of those systems. The official stressed that all eight Himars inside Ukraine are accounted for and still in use by Ukrainian forces.
"Ukraine has now been successfully striking Russian locations in Ukraine, deeper behind the front lines and disrupting Russia's ability to conduct that artillery operation," the official said.
The latest weapons package also includes more precise ammunition for howitzer artillery systems, three tactical vehicles, counter-battery systems and spare parts.
Despite Russia’s advances in Eastern Ukraine’s Luhansk region, the Pentagon said the Ukrainians are defending “tenaciously”.
“All along the way they [Ukrainian forces] were inflicting significant casualties on the Russians,” the official said. The high level of casualties is forcing Moscow to “reorganise”, the US said.
“I can't imagine taking casualties on the level the Russians have taken them and not have a morale problem,” the official said.
The US latest aid package to Ukraine comes as Mr Biden visited the Central Intelligence Agency on Friday.
The visit is intended to commemorate the agency’s 75th anniversary since its founding after World War II. But Mr Biden met with CIA officers who have worked on Ukraine.
"You are clearly the best in the whole world," Mr Biden told CIA staff.
"It was thanks to the incredible work of our intelligence professionals that we were able to forewarn the world with what Vladimir Putin was planning in Ukraine."
Those leaks "punched a gigantic hole in the pretence and discredited his [Putin's] lies," Mr Biden said from Langley.
Largely accurate predictions that Russia would invade Ukraine in February generated a public boost for US spy agencies that are often criticised and facing new pressure to deliver insights on China and Russia.
The Associated Press contributed to this report
