US Secretary of State Antony Blinken said Wednesday 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," Blinken 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 Tuesday 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.

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," Blinken 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 we want to make sure that Russia is feeling strong pressure from as many countries as possible to end the aggression," he said.

Blinken dismissed suggestions that the United States was risking escalation with Russian President Vladimir Putin, who invaded Ukraine in February despite repeated Western warnings.

"It is Russia that is attacking Ukraine, not the other way around," Blinken said.

"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."