BRUSSELS — NATO officials, determined to show a unified front of support for Ukraine, on Friday accused Russia of using cluster bombs in its invasion, but rejected Kyiv’s plea to impose a no-fly zone over Ukrainian airspace, fearing that would draw the military alliance into a larger war with Russia.

Dozens of protesters waving Ukrainian flags chanted “NATO, act now!” outside the headquarters of the alliance as its foreign ministers met in a special session focused on helping Ukraine repel Russia’s expanding invasion. Ministers also met in sessions organized by the European Union and the Group of 7.

Inside the NATO headquarters, the ministers heard an appeal from Ukraine’s foreign minister, Dmytro Kuleba, asking for more military assistance than NATO has so far been willing to give.

“Help us,” Mr. Kuleba pleaded in a video that was posted on Twitter. “If you don’t, I’m afraid you will have to share responsibility for the lives and the suffering of civilian Ukrainians, who die because of ruthless Russian pilots who throw bombs on them.”

Shortly afterward, the NATO secretary general, Jens Stoltenberg, confirmed that Russia had attacked Ukraine with anti-personnel cluster bombs, which kill so indiscriminately they are banned under international law.

But he rejected Mr. Kuleba’s plea for a no-fly zone. “Allies agree that we should not have NATO planes operating over Ukrainian airspace or NATO troops on Ukrainian territory,” he said.

Cluster bombs can be dropped from planes or launched from rockets. Enforcing a no-fly zone usually requires warplanes to patrol airspace, with troops below to identify and report violations.

The day of meetings of the NATO, E.U. and G7 foreign ministers was billed as a display of Western unity in response to Russia’s war on Ukraine. Even Liz Truss, the British foreign secretary, set Brexit aside to preach “complete unity” at the E.U. meeting, which the U.S. secretary of state, Antony J. Blinken, also attended.

“We are faced together with what is President Putin’s war of choice — unprovoked, unjustified,” Mr. Blinken told journalists ahead of the meeting. “We’re committed to doing everything we can to make it stop. So the coordination between us is vital.”

The crisis has reminded Europeans of how much they depend on the United States for leadership and military muscle, as well as the nuclear umbrella that serves as the most important element of deterrence against countries like Russia, China or even Iran.

And if the European Union, under a French presidency, chafed about America’s leading role at the start of the crisis, President Biden and Mr. Blinken have gone to great lengths to inform and consult with the Brussels institutions as well as member states.

But as the crisis unfolded, Washington provided convincing intelligence to its allies and organized the response. It brought the European Union, Britain, Canada and Australia along on a tough package of economic sanctions and was quick to start supplying weapons to Ukraine and to move troops and matériel to shore up allied forces along NATO’s eastern flank.

“While our focus should remain on Ukraine’s sovereignty and the restoration of Ukrainians’ safety, I believe Putin’s war will also elevate America’s global standing,” wrote Kori Schake, a former American defense official. “Indeed, it has already strengthened America’s position at the center of the international order that it created from the ashes of World War II.”

Other countries also played a key role, with France volunteering to lead a new NATO battalion in Romania and with Germany mothballing Nord Stream 2, the gas pipeline from Russia to Western Europe, and allowing the export of weapons to Ukraine. The European Union, too, moved to “Europeanize” the efforts of many of its 27 member states — 21 of which belong to NATO — and promised for the first time to reimburse them for weapons sent to Ukraine.

Whether more weapons should have been sent to Ukraine in the long buildup to the war remains an open question, but NATO officials were concerned that an open display of weapons being supplied might provoke Russia, rather than deter it.

With weapons and supplies coming from Poland and other neighboring countries into western Ukraine, there are concerns that Russia will move to block or bomb convoys and that there could be accidental confrontations with NATO planes.

Mr. Blinken’s visit, in large part, sought to keep the European Union in sync with the Western drumbeat of economic sanctions — with possibly more to come — to punish the government of President Vladimir V. Putin and his allies.

He also rallied allies to send more humanitarian support to relief workers in Ukraine and its neighboring states, including to Poland and Moldova, where he will hold meetings on Saturday. More than one million refugees have fled Ukraine over the last week, and at least 100,000 more who remain in Ukraine have been forced from their homes in the fighting.

Josep Borrell Fontelles, the E.U. foreign policy chief, said Russia had bombed and shelled houses, schools and hospitals.

“This is a barbarian way of doing war,” Mr. Borrell said.

Mr. Stoltenberg said that in addition to cluster bombs, Russia has used other banned weapons in Ukraine.

“We have seen the use of cluster bombs, and we have seen reports of use of other types of weapons which would be in violation of international law,” he said. He did not specify any other kind of weapon.

Anti-personnel cluster munitions are rockets, missiles, artillery shells and bombs that deploy a large number of small explosives over a wide area, intended to attack infantry formations. The Convention on Cluster Munitions, a treaty banning such weapons, took effect in August 2010.

NATO forces used cluster bombs during the Kosovo war in 1999, and the United States dropped more than 1,000 cluster bombs in Afghanistan from October 2001 to March 2002, according to a Human Rights Watch report.