WASHINGTON (AP) — The U.S. and allies committed more rocket systems, ammunition and other military aid to Ukraine Wednesday, as American defense leaders said they see the war to block Russian gains in the eastern Donbas region grinding on for some time.

Speaking at the close of a virtual meeting with about 50 defense leaders from around the world, U.S. Defense Secretary Lloyd Austin said it will be “hard work” to keep allies and partners all committed to the war effort as the months drag on.

“We’re pushing hard to maintain and intensify the momentum of donations,” Austin said. “This will be an area of focus for the foreseeable future, as it should be, in terms of how long our allies and partners will remain committed ... There’s no question that this will always be hard work making sure that we maintain unity.”

Officials have been reluctant to say how long the war may last, but Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff, suggested it could be a long slog.

“We have a very serious grinding war of attrition going on in the Donbas. And unless there’s a breakthrough on either side — which right now the analysts don’t think is particularly likely in the near term — it will probably continue as a grinding war of attrition for a period of time until both sides see an alternative way out of this, perhaps through negotiation or something like that.”

Officials said Wednesday that the U.S. will send Ukraine four more High Mobility Artillery Rocket Systems (HIMARS) and precision-guided rockets for them, as well as additional artillery rounds. A more detailed announcement is expected later this week.

The aid comes as Russian forces try to solidify gains in the two provinces in Ukraine's eastern Donbas region, Donetsk and Luhansk, while also expanding attacks into other areas. Russian Foreign Minister Sergey Lavrov told state-controlled RT television and the RIA Novosti news agency that Russia has expanded its “special military operation” from the Donbas to the Kherson and Zaporizhzhia regions and other captured territories.

Austin said Lavrov's comments come as no surprise to allies who have known Russia has greater ambitions in capturing Ukraine.

But Ukrainian troops have been using the HIMARS to strike Russian logistics nodes and command and control centers, including behind the front lines to disrupt supply chains. And on Wednesday they struck and damaged a bridge that is key to supplying Russian troops in southern Ukraine, where Lavrov said Moscow is trying to consolidate its territorial gains.

Milley said the Ukrainian strikes are “steadily degrading the Russian ability to supply their troops, command and control their forces, and carry out their illegal war of aggression.”

He said that, due to Ukraine's resistance, Russia has been able to gain just six to 10 miles of ground in the Donbas over the past 90 days, with “tens of thousands of artillery rounds” fired in each 24-hour period. And he said he does not believe that the Donbas region has been lost to Russia.

“It’s not lost yet. The Ukrainians are making the Russians pay for every inch of territory that they gain and advances are measured in literally hundreds of meters,” Milley said.

The issue going forward, he said, will be the amount of HIMARS rockets and other ammunition expended by the Ukraine forces. The U.S. has been sending thousands of rounds, taking them from American military stockpiles, and raising questions about how long that will last and at what point there may be a risk to U.S. military readiness.

“We are looking at all of that very, very carefully,” Milley said. “We think we’re okay right now as we project forward into the next month or two or three, we think we’re going to be okay.”

The U.S. has already provided more than $7 billion in aid to Ukraine since the war began in late February. Austin said that during the defense meeting, there was also discussion about how to ensure that Ukraine is able to maintain and repair the weapons systems into the future.

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