US Secretary of State Antony Blinken said that Ukraine had promised not to use new long-range missiles to strike inside of Russia as he warned of "many months" of conflict ahead.
"The Ukrainians have given us assurances that they will not use these systems against targets on Russian territory," he told a joint news conference with NATO Secretary-General Jens Stoltenberg.
"There is a strong trust bond between Ukraine and the United States, as well as with our allies and partners," he said.
The United States said yesterday that it will supply Ukraine with advanced missile systems, including the Himars multiple-launch rocket system that can simultaneously launch multiple precision-guided missiles.
President Joe Biden said that the United States would not support attacks inside of Russia, which quickly warned of greater risks of conflict between Washington and Moscow.
Russian Foreign Minister Sergei Lavrov, speaking in Saudi Arabia, said the supply of the rocket launchers raised the risks of a "third country" being dragged into the conflict.
Kremlin spokesman Dmitry Peskov said such supplies would not encourage Ukraine's leadership to resume stalled peace talks.
"We believe that the United States is purposefully and diligently adding fuel to the fire," he said.
Mr Blinken said the United States was arming Ukraine in anticipation of a long conflict ahead.
"As best we can assess right now, we are still looking at many months of conflict," he said.
"That could be over tomorrow if Russia chose to end the aggression. We don't see any signs of that right now," he said.
"As long as this goes on, we want to make sure that Ukraine has in hand what it needs to defend itself and that Russia is feeling strong pressure from as many countries as possible to end the aggression," he said.
Mr Blinken dismissed suggestions that the United States was risking escalation with Russian President Vladimir Putin, who invaded Ukraine in February despite repeated Western warnings.
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"It is Russia that is attacking Ukraine, not the other way around," he added.
"And simply put, the best way to avoid escalation is for Russia to stop the aggression and the war that it started. It's fully within its power to do so."
Russian forces today pressed closer to the centre of an industrial city in a drive to grab a swathe of eastern Ukraine.
Ukraine said that Russian forces, now 98 days into their invasion, had pounded infrastructure in eastern and southern regions, including the city of Sievierodonetsk.
They entered this on 27 May and it is the main focus of their ground offensive in the eastern Donbas region.
According to provincial governor Serhiy Gaidai, a Russian air strike hit the Azot chemical factory in Sievierodonetsk yesterday, blowing up a tank of toxic nitric acid and releasing a plume of pink smoke.
He urged residents to remain inside.
Ukrainian forces now control just 20% of Sievierodonetsk, Russian forces 60% and the rest has become a "no-man's land", Oleksandr Stryuk, the Ukrainian head of the city administration, told Reuters.
"The 20% is being fiercely defended by our armed forces...Attempts are being made to drive out the Russian troops... We have hope that despite everything we will free the city...," Mr Stryuk said.
Governor Gaidai said Lysychansk was easier to defend as it is located on a hill but Russian forces would target it with artillery and mortars once in full control of Sievierodonetsk.
The leader of the pro-Moscow Luhansk People's Republic, Leonid Pasechnik, told TASS news agency Russian proxies had advanced slower than expected to safeguard city infrastructure and "exercise caution around its chemical factories".
Ukrainian President Volodymyr Zelensky has called for more weapons while lambasting the European Union - which agreed on Monday to cut imports of Russian oil - for not sanctioning energy from Russia sooner.
The EU will ban imports of Russian oil by sea. Officials said that would halt two-thirds of Russia's oil exports to Europe at first, and 90% by the end of this year.
Responding to the EU oil embargo, Russia widened its gas cuts to Europe, driving prices higher and ratcheting up its economic battle with Brussels.
Vladimir Putin launched what he calls a special military operation on 24 February to disarm and "denazify" Ukraine. Ukraine and its Western allies call this a baseless pretext for a war of aggression.
