President Biden’s deliberations with Prime Minister Keir Starmer of Britain about whether to allow Ukraine to attack Russia with long-range Western weapons were fresh evidence that the president remains deeply fearful of setting off a dangerous, wider conflict.

But the decision now facing Mr. Biden after Friday’s closed-door meeting at the White House — whether to sign off on the use of long-range missiles made by Britain and France — could be far more consequential than previous concessions by the president that delivered largely defensive weapons to Ukraine during the past two and a half years.

In remarks at the start of his meeting with Mr. Starmer, the president underscored his support for helping Ukraine defend itself but did not say whether he was willing to do more to allow for long-range strikes deep into Russia.

“We’re going to discuss that now,” the president told reporters.

For his part, the prime minister noted that “the next few weeks and months could be crucial — very, very important that we support Ukraine in this vital war of freedom.”

European officials said earlier in the week that Mr. Biden appeared ready to approve the use of British and French long-range missiles, a move that Mr. Starmer and officials in France have said they want to provide a united front in the conflict with Russia. But Mr. Biden has hesitated to allow Ukraine to use arms provided by the United States in the same way over fears that President Vladimir V. Putin of Russia would see it as a major escalation.

On Thursday, Mr. Putin responded to reports that America and its allies were considering such a move by declaring that it would “mean that NATO countries — the United States and European countries — are at war with Russia,” according to a report by the Kremlin.

Mr. Biden and Mr. Starmer offered little insight on Friday into the actions they planned to take. But officials on both sides of the Atlantic said they did not expect any announcement immediately after the White House meeting. In the past, Western countries have begun providing new military equipment to Ukraine without announcing the decision publicly.

“This wasn’t about a particular decision that we’ll obviously pick up again in UNGA in just a few days’ time with a wider group of individuals,” Mr. Starmer told reporters after the meeting, referring to the annual meeting in New York of the United Nations General Assembly at the end of the month.

But he also hinted that he expected a decision about the missiles to come soon.

“I think if you look at both the Ukrainian situation and the Middle East, it is obvious that in the coming weeks and months there are really important potential developments, whatever timetable is going on in other countries,” he said.

John F. Kirby, the national security spokesman at the White House, said Friday that the Biden administration takes Mr. Putin’s threats seriously because he has proved himself capable of “aggression” and “escalation.” But Mr. Kirby added that there had been no change in Mr. Biden’s opposition to letting Ukraine use U.S. missiles to strike deep inside Russia.

“There is no change to our view on the provision of long-range strike capabilities for Ukraine to use inside Russia, and I wouldn’t expect any sort of major announcement in that regard coming out of the discussions, certainly not from our side,” he said.

Mr. Kirby’s comments came just hours before the two leaders met for their first lengthy conversation since Mr. Starmer became prime minister in early July.

The question of whether to let Ukraine use the long-range weapons that can travel 150 to 200 miles has been a rare point of disagreement between British and American officials, who have largely been in lock step on strategy over the past 30 months of fighting.

British officials have argued that Ukraine cannot be expected to fight effectively unless it can attack the military sites that Russia is using to shoot missiles or the airplanes that deliver “glide bombs.” And they believe that Mr. Putin, for all his nuclear threats warning that war between Russia and European forces could be coming, is largely bluffing. Mr. Putin, they say, has shown he does not want to bring NATO directly into the fighting.

Mr. Biden’s view has been far more cautious.

He has hesitated at every major decision point, starting with shipping HIMARS artillery, then through debates on whether to send M1 Abrams tanks, F-16 fighters, and short- and long-range ATACMS, a missile system critical to American preparations to defend both Europe and the Korean Peninsula.

But those decisions have primarily helped Ukraine’s military defend its territory and try to repel the Russian invasion. Over time, his aides say, they have discovered that Mr. Putin was less sensitive to the introduction of new weapons into the battlefield than they had thought. So they have gradually approved more capable, longer-range arms for Ukraine.

The questions of how Mr. Putin would react to the use of American weapons by Ukraine to strike deep inside Russian territory, officials say, could lead to a very different outcome.

“When he starts brandishing the nuclear sword, for instance, yeah, we take that seriously, and we constantly monitor that kind of activity,” Mr. Kirby said. “We have our own calculus for what we decide to provide to Ukraine and what not.”

The American concerns are twofold. The first has been rooted in Mr. Biden’s concern that the war not escalate; time and again he has told members of his staff that their No. 1 priority was to “avoid World War III.”

The second American concern is a practical one: Pentagon officials do not believe Ukraine has enough of the ATACMS, the British Storm Shadow and the French SCALP missiles to make a strategic difference on the battlefield. The reach of the missiles, they note, is well known — and Russia has already moved its most valuable aircraft beyond the range the missiles can fly.

Moreover, the U.S. officials say, they simply cannot supply many more to Ukraine. The Pentagon has warned that it must keep a healthy reserve of weapons in case of an outbreak of fighting in either Europe or Asia. And the missiles are so expensive that they contend Ukraine could get more firepower putting that money into drones.

So in the American telling of events, the decisions being debated by Mr. Biden and Mr. Starmer are more symbolic than substantive.

Looming over this is the American election.

In the debate against Vice President Kamala Harris on Tuesday, former President Donald J. Trump declined several opportunities to say he was committed to Ukraine’s victory. Instead, he talked of striking a deal, one that Ukraine may be coerced to sign.

While Ms. Harris is likely to continue the outlines of the American strategy, providing more arms and aid to Ukraine as long as Congress keeps the spigot open, Mr. Trump has made clear he is uninterested in continuing to spend heavily. And while Europe has stepped up, it does not have enough of an arsenal to make much of a difference.