The weapons were part of a $300m (£240m) aid package approved by US President Joe Biden in March and they arrived this month.

They have already been used at least once to strike Russian targets in occupied Crimea, US media report.

Mr Biden has now signed a new $61bn package of aid for Ukraine.

The US previously supplied Ukraine with a mid-range version of the Army Tactical Missile Systems (ATACMS) but had been reluctant to send anything more powerful, partly over concerns about compromising US military readiness.

However, Mr Biden is said to have secretly given the green light to send the long-range system - which can fire missiles distances of up to 300km (186 miles) - in February.

"I can confirm that the United States provided Ukraine with long-range ATACMS at the president's direct direction," state department spokesman Vedant Patel said.

The US "did not announce this at the onset in order to maintain operational security for Ukraine at their request", he added.

It is not clear how many of the weapons have already been sent, but US National Security Advisor Jake Sullivan said Washington planned to send more.

"They will make a difference. But as I've said before at this podium... there is no silver bullet," he said.

The longer-range missiles were used for the first time last week to strike a Russian airfield in occupied Crimea, Reuters news agency quoted an unnamed US official as saying.

And the new missiles were also used in an attack on Russian troops in the occupied port city of Berdyansk overnight on Tuesday, according to the New York Times.

Recent months have seen Kyiv step up its calls for Western assistance as its stocks of ammunition are depleted and Russia makes steady gains.

The new aid package follows months of gridlock amid opposition to the aid from some in Congress.

"It's going to make America safer, it's going to make the world safer," Mr Biden said after signing it into law.

Reacting to the package, Ukrainian President Volodymyr Zelensky said: "Now we will do everything to make up for half a year spent in debates and doubts.

"What the Russian occupier was able to do during this time, what Putin is now planning, we must turn against him."

Mr Zelensky recently warned that a Russian offensive was expected in the coming weeks after Ukraine's loss of the city of Avdiivka during the winter.

Ukrainian forces have suffered from a shortage of ammunition and air defence systems in recent months and officials have blamed delays in military aid from the US and other Western allies for the loss of lives and territory.

Mr Sullivan said on Wednesday it was "certainly possible that Russia could make additional tactical gains in the coming weeks".

Since Russia invaded Ukraine on 24 February 2022, tens of thousands of people, most of them soldiers, have been killed or injured on both sides and millions of people have fled their homes.

By Kathryn Armstrong