The weapons, which can hit targets as far as 300 kilometres away, have been used twice already.
The United States quietly sent long-range ballistic missiles to Ukraine as part of a package of military support in March, and Ukraine has used the weapons twice, according to US officials.
The longer-range Army Tactical Missile Systems (ATACMS) can hit targets as far as 300km (186 miles) away, nearly double the range of the mid-range ATACMS that the US began sending towards the end of last year.
Washington had long been reluctant to provide Ukraine with the longer-range weapon amid concerns they could be used on targets deep inside Russian territory and escalate the conflict.
But in February, US President Joe Biden approved the delivery of the missiles and a âsignificantâ number was included in a $300m aid package announced the following month, officials said.
âWeâve already sent some, we will send more,â White House national security adviser Jake Sullivan told reporters.
US Department of State spokesman Vedant Patel said the delivery had not been announced âin order to maintain operational security for Ukraine at their requestâ. Neither official confirmed the number of ATACMS sent.
On Thursday, Russia said the deliveries of ATACMS would not change the outcome of the war.
âThe US is directly involved in this conflict. They are following the path of increasing the operating range of the weapon systems they supply,â Kremlin spokesman Dmitry Peskov told reporters.
âWe will achieve our goal,â he said, adding that âthis will cause more problems for Ukraine itselfâ.
Ukraine has been forced to ration its weapons amid a protracted delay to a $61bn military assistance package that was finally passed in the US this week. ATACMS are expected to be included in the first $1bn tranche of that aid package.
The weapons sent this month were used on April 17 to strike an airfield in Dzhankoi in Crimea, which Russia seized from Ukraine in 2014. They were also used this week against Russian forces in southeastern Ukraine, near the occupied city of Berdyansk.
âTime is rightâ
Admiral Christopher Grady, vice chairman of the Joint Chiefs of Staff, said the White House and military planners had looked carefully at the risks of providing long-range weaponry to Ukraine and determined that it was the right time.
The weapons were sent on the condition they be used only inside Ukrainian sovereign territory.
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âI think the time is right, and the boss [Biden] made the decision the time is right to provide these based on where the fight is right now,â Grady said.
A US official told the Reuters news agency it was Russiaâs use of North Korean-supplied long-range ballistic missiles against Ukraine in December and January that led to the change of heart.
Russiaâs continued targeting of Ukraineâs critical infrastructure was also a concern.
âWe warned Russia about those things,â said the official, who spoke on condition of anonymity. âThey renewed their targeting.â
The delay in US funding and weapons deliveries has given Russia the space to push its advantage in firepower and personnel to step up attacks across the front line in eastern Ukraine, where it claims to have taken control of a number of settlements this month. It has increasingly used satellite-guided gliding bombs â dropped from planes at a safe distance â to pummel Ukrainian forces.
Ukrainian officials have not publicly acknowledged the receipt or use of long-range ATACMS.
But in thanking Congress for passing the new aid bill, Ukrainian President Volodymyr Zelenskyy stressed the significance of such weaponry to the war effort.
âUkraineâs long-range capabilities, artillery and air defense are extremely important tools for the quick restoration of a just peace,â he wrote on social media platform X.