The United States is expected to provide an additional $450 million in security assistance to Ukraine, including more long-range rocket systems, two U.S. officials told Reuters on Thursday.

Moscow’s forces are advancing in Ukraine’s east in a bid to capture the industrial heartland known as the Donbas, where Ukraine fears some of its troops could be encircled in a Russian pincer move.

One of the officials, speaking on the condition of anonymity, said an announcement was expected later on Thursday and the latest package is expected to include four additional High Mobility Artillery Rocket Systems (HIMARS).

The official added that details on the package could change at the last minute.

Earlier on Thursday, Ukraine said it had received the first tranche of HIMARS, a powerful long-range weapon system that Kyiv hopes can help turn the tide in the months-long battle.

Ukraine says it needs the HIMARS systems to better match the range of Russian rocket systems that it says are being extensively used to pummel Ukrainian positions in Donbas.

Washington has said it has received assurances from Kyiv that those longer-range weapons would not be used to attack Russian territory, fearing an escalation of the conflict.

Moscow has warned it will strike targets in Ukraine which they "have not yet been hitting" if the West supplies longer-range missiles to Ukraine for use in high-precision mobile rocket systems.