The Biden administration has announced another $1bn in new military aid for Ukraine.
The US is pledging what will be the biggest yet delivery of rockets, ammunition and other arms straight from Department of Defense stocks for Ukrainian forces.
This pledge comes as analysts warned that Russia was moving troops and equipment in the direction of the southern port cities to prevent a Ukrainian counter-offensive.
Included in the aid will be additional rockets for the High Mobility Artillery Rocket Systems, or HIMARS, as well as thousands of artillery rounds, mortar systems, Javelins and other ammunition and equipment.
Military commanders and other US officials say the HIMARS and artillery systems have been crucial in Ukraine's ongoing fight to try to prevent Russia from taking more ground.
In July, the UK also announced it is to supply Ukraine with scores of artillery guns, hundreds of drones and anti-tank weapons.
Defence Secretary Ben Wallace told MPs more than 20 M109 155mm self-propelled guns and 36 L119 105mm artillery guns will soon be delivered to the wartorn country to add to what military equipment the UK has already sent Ukraine.
The delivery of the guns will be followed by counter-battery radar systems and more than 50,000 rounds of ammunition suitable for Ukraine's existing artillery weapons, Mr Wallace said.
The latest US announcement brings the total US security assistance committed to Ukraine by the Biden administration to roughly $9bn since Russian troops invaded in late February.
"At every stage of this conflict, we have been focused on getting the Ukrainians what they need, depending on the evolving conditions on the battlefield," Colin Kahl, undersecretary of defence for policy, said in announcing the new weapons shipment.
Watch: Why Ukraine wants more HIMARS weapons.
Until now, the largest single security assistance package announcement was for $1bn on 15 June.
But that aid included $350m in presidential drawdown authority and another $650m under the Ukraine Security Assistance Initiative.
This most recent package allows the US to deliver weapons systems and other equipment more quickly since it takes them off the Defence Department shelves.
For the last four months of the war, Russia has concentrated on capturing the Donbas region of eastern Ukraine, where pro-Moscow separatists have controlled some territory as self-proclaimed republics for eight years.
Russian forces have made gradual headway in the region while launching missile and rocket attacks to curtail the movements of Ukrainian fighters elsewhere.
Dmytro Kuleba the Minister of Foreign Affairs of Ukraine said on Twitter, that he was "deeply grateful" to the US ally.
He thanked US Secretary Antony Blinken for the "timely and essential security package that will help us defeat the ongoing Russian aggression".
"This move proves that Ukraine and the US enjoy a high level of trust and co-operation," he said.