Increasing number of pictures are appearing on Ukrainian social media showing a US-made rocket launcher being used against Russian targets.
At the start of June, the US Department of Defence said it would be sending four HIMAR systems to Ukraine to help the country defend itself from attack.
It said Ukrainian soldiers would be trained on it first, which would take about three weeks.
In the last few days, there have been a number of social media posts from Ukrainian military sources that have showed the HIMARS in action in the country, reportedly targeting forces from Russia.
One shows vehicles travelling along a road purportedly in southern-eastern Zaporizhzhia region and then the system in use, launching a volley of missiles into a twilit night, was put out by the official account of the General Staff of the Ukrainian Armed Forces.
The tweet said in English as well as Ukrainian: "Combat work of HIMARS. Zaporizhzhia direction. Beautiful, fast, accurate. @DeptofDefense @thejointstaff".
Another, all in Ukrainian, shows the vehicle in operation at night, lighting up the sky as the rockets are launched, while another, from a different source, shows the equipment being reloaded with a "cartridge" of another six rockets and their firing tubes.
A tweet by an open source enthusiast calling themselves The Cube, who says they have analysed the scene where some of the rockets were fired from, claims they were aimed at Russian positions around Melitopol airport.
It is thought the HIMARS has arrived close to the frontline in the last few days as on 22 June, the Ukrainian armed forces said they were still waiting for the US rocket launch system to arrive.
It comes as other social media pictures put out by Ukrainian defence sources are increasingly showing the use of other Western weapons such as the French 155mm CAESAR self-propelled heavy artillery.
Analysts say that while it could be the start of the arrival of a number of Western weapons that had been promised to Ukraine for several months that have the potential to turn the tide of the war, more will be needed before it starts to make a difference.
Forbes McKenzie, an defence intelligence analyst, told Sky News: "The artillery pieces that are now coming into action: the CAESAR system, a 155mm artillery piece from the French, has been brought into action... the HIMAR system from the US... they need a combat mass of 54 to have real effect.
"They can fire six rockets at any one time, be in action in 16 seconds, they can shoot, they can scoot, they can survive. They are the type of munitions the Ukrainians will need in order to deter and defeat the Russians in the coming months.
"This was used at the weekend. On 2 July an airport in the Melitopol region had a lot of equipment on it from the Russians. We understand that on the 3rd, this weapon system was used against those aircraft.
"It's starting to come into use but it will take a period of months to commit properly."
The US Department of Defence, which has promised to send four more HIMARS in the next couple of weeks, says the rockets supplied to Ukraine have a range of 40 miles.
In past use, however, it has been said to be effective up to a maximum range of 300km (187 miles).
Colin Kahl, US undersecretary of defence for policy, told a Pentagon briefing on 1 June: "These are precision guided systems with extended range, for high value targets, that allows them to keep some of the pressure off of Ukrainian forces on the front."
There have been other unverified claims that the HIMARS has been used in the last few days against a Russian base in the Izyum area.
HIMARS isn't the only rocket launch system that is being sent to Ukraine. A system called the M270, developed by a number of NATO countries including the UK, is being supplied by British forces and Norway.
It is thought that is yet to arrive.