WASHINGTON – The U.S. will send the Ukrainian military four sophisticated, medium-range rocket systems and ammunition to help try to stall Russian progress in the Donbas region of its country, but it will take at least three weeks to get the precision weapons and trained troops onto the battlefield, the Pentagon said Wednesday.

Colin Kahl, the defense undersecretary for policy, said the U.S. has received assurances at the highest government levels that Ukraine will use the rockets to defend its nation and not launch them into Russia. The agreement underscores U.S. concerns about provoking a wider war with Russia while still providing Ukraine the weapons it has desperately requested in recent weeks.

The rocket systems are part of a new $700 million tranche of security assistance for Ukraine from the U.S. that also includes helicopters, Javelin anti-tank weapon systems, radars, tactical vehicles, spare parts and more.

“It is a grinding fight,” he said during a Pentagon briefing. “We believe that these additional capabilities will arrive in a timeframe that’s relevant and allow the Ukrainians to very precisely target the types of things they need for the current fight.”

The U.S. decision to provide the advanced rocket systems tries to strike a balance between helping Ukraine battle ferocious Russian artillery barrages and not providing arms that could allow Ukraine to hit targets deep inside Russia and trigger an escalation in the war.

In a guest essay published Tuesday evening by The New York Times, President Joe Biden confirmed that he would “provide the Ukrainians with more advanced rocket systems and munitions that will enable them to more precisely strike key targets on the battlefield in Ukraine.”

Biden had said Monday that the U.S. would not send Ukraine “rocket systems that can strike into Russia.” Any weapons system can shoot into Russia if it’s close enough to the border. The aid package unveiled Wednesday would send what the U.S. considers medium-range rockets — they generally can travel about 45 miles, officials said.

“The Ukrainians have given us assurances that they will not use these systems against targets on Russian territory,” Secretary of State Antony Blinken said Wednesday. “There is a strong trust bond between Ukraine and the United States.”

Kremlin spokesman Dmitry Peskov told reporters Wednesday that the U.S. is “deliberately and diligently pouring fuel on the fire.” He added that the Kremlin doesn’t trust Kyiv’s assurances that the multiple rocket launch systems supplied by the U.S. will not be used to attack Russia.

“In order to trust (someone), you need to have experience with situations when such promises were kept. Regretfully, there is no such experience whatsoever,” Peskov said.

The expectation is that Ukraine could use the rockets in the eastern Donbas region, where they could both intercept Russian artillery and take out Russian positions in towns where fighting is intense, such as Sievierodonetsk.

Sievierodonetsk is important to Russian efforts to capture the Donbas before more Western arms arrive to bolster Ukraine’s defense. The city is in an area that is the last pocket under Ukrainian government control in the Luhansk region of the Donbas.

Biden added in his New York Times essay: “We are not encouraging or enabling Ukraine to strike beyond its borders. We do not want to prolong the war just to inflict pain on Russia.”

It’s the 11th package approved so far and will be the first to tap the $40 billion in security and economic assistance recently passed by Congress. The rocket systems would be part of Pentagon drawdown authority, meaning the arms transfer would involve taking weapons out of existing U.S. inventory and getting them into Ukraine quickly. Ukrainian troops would also need training on the new systems, which could take at least a week or two.

Officials said the plan is to send Ukraine the High Mobility Artillery Rocket System, or HIMARS, which is mounted on a truck and can carry a container with six rockets. The system can launch a medium-range rocket, which is the current plan, but is also capable of firing a longer-range missile, the Army Tactical Missile System, which has a range of about 190 miles and is not part of the plan.

Since the war began in February, the U.S. and its allies have tried to walk a narrow line: send Ukraine weapons needed to fight off Russia, but stop short of providing aid that will inflame Russian President Vladimir Putin and trigger a broader conflict that could spill over into other parts of Europe.

Over time, however, the U.S. and allies have amped up the weaponry going into Ukraine. To that end, Ukrainian President Volodymyr Zelenskyy has been pleading with the West to send multiple launch rocket systems to Ukraine as soon as possible to help stop Russia’s destruction of towns in the Donbas. The rockets have a longer range than the howitzer artillery systems that the U.S. has provided Ukraine. They would allow Ukrainian forces to strike Russian troops from a distance outside the range of Russia’s artillery systems.

“We are fighting for Ukraine to be provided with all the weapons needed to change the nature of the fighting and start moving faster and more confidently toward the expulsion of the occupiers,” Zelenskyy said in a recent address.

Ukraine needs multiple launch rocket systems, said Philip Breedlove, a retired U.S. Air Force general who was NATO’s top commander from 2013 to 2016.

“These are very important capabilities … and they not only need them, but they have been very vociferous in explaining they want them,” said Breedlove. “We need to get serious about supplying this army so that it can do what the world is asking it to do: fight a world superpower alone on the battlefield.”

Russia has been making incremental progress in the Donbas, as it tries to take the remaining sections of the region not already controlled by Russian-backed separatists.