The weapons, which can hit targets as far as 300km (186 miles) away, have been used twice already.

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, Biden approved the delivery of the missiles and a “significant” number was included in a $300m aid package announced the following month, officials said.

“We’ve already sent some, we will send more,” White House national security adviser Jake Sullivan told reporters.

State Department spokesman Vedant Patel said the delivery had not been announced “in order to maintain operational security for Ukraine at their request”. Neither official confirmed the number of ATACMS sent.

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.

‘Time is right’

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.

“I think the time is right, and the boss [President Biden] made the decision the time is right to provide these based on where the fight is right now,” Grady said.

A US official told the Reuters news agency it was Russia’s use of North Korean-supplied long-range ballistic missiles against Ukraine in December and January that led to the change of heart.

Russia’s continued targeting of Ukraine’s critical infrastructure was also a concern.

“We warned Russia about those things,” said the official, who spoke on condition of anonymity. “They renewed their targeting.”

The protracted 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 – dropped from planes at a safe distance – 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.

“Ukraine’s long-range capabilities, artillery and air defense are extremely important tools for the quick restoration of a just peace,” he wrote on social media platform X.