The missiles were contained in a $US300 million ($A462 million) military aid package for Ukraine that US President Joe Biden approved on March 12, said the US official, speaking on condition of anonymity.
The official would not say how many of the missiles were sent.
The missiles were used for the first time in the early hours of April 17, launched against a Russian airfield in Crimea that was about 165km from the Ukrainian front lines, the official said.
The official said Ukraine used the weapon a second time overnight against Russian forces in southeastern Ukraine.
Whether to send the Army Tactical Missile Systems (ATACMS) with a range up to 300km was a subject of debate within the US administration for months.
Mid-range ATACMS were supplied last September.
The Pentagon initially opposed the long-range missile deployment, fearing the loss of the missiles from the US stockpile would hurt the country's military readiness.
US authorities were initially concerned Ukraine would use the new missiles to attack inside Russia. (AP PHOTO)
There were also concerns that Ukraine would use them to attack targets deep inside Russia.
Russia's use of North Korean-supplied long-range ballistic missiles against Ukraine in December and January, despite US public and private warnings not to do so, led to a change in heart, the US official said.
Also a factor in US decision-making was Russia's targeting of Ukraine's critical infrastructure, the official said.
"We warned Russia about those things," the official said.
"They renewed their targeting."
In late January the US military found a way to satisfy their concerns about military readiness, which enabled the administration to move forward.
They began acquiring new missiles coming off the Lockheed-Martin production line.
Biden met with his national security team in mid-February and agreed to accept the unanimous recommendation of his advisers to send the missiles to Ukraine.
Involved in the discussion were national security adviser Jake Sullivan, Defense Secretary Lloyd Austin, Secretary of State Antony Blinken and Joint Chiefs of Staff chairman CQ Brown.
The challenge at that point was to figure out how to pay for the missiles.
The United States had exhausted all of its funding options and congressional gridlock stymied further aid.
An opportunity arose in March, when several Pentagon contracts came in under bid.
Biden was able to use the difference to send $US300 million in assistance to Ukraine.
Biden told his team to include the long-range ATACMS in this funding package but to do so secretly in order to maintain operational security and the element of surprise for Ukraine, the official said.
