Russian forces launched a deadly strike on a city in eastern Ukraine on Saturday morning as President Volodymyr Zelensky marked the 500th day of the war with a show of defiance, sharing a video of himself visiting Snake Island, a strip of land in the Black Sea that has become a potent symbol of his country’s resistance to the invasion.

At least eight civilians were killed and 15 others were injured when Russian forces shelled a residential area of the city of Lyman at around 10 a.m., Ukraine’s Interior Ministry said in a statement on the Telegram messaging app. Rescue efforts continued, according to officials.

The midmorning attack was a grim reminder of the toll taken on Ukraine by 500 days of war. Mr. Zelensky paid tribute on Saturday to all those who have lost their lives, using the backdrop of Snake Island to underscore Ukrainian resolve.

At the start of the war in February, an audio recording captured Ukrainian border guards on the island, which is 20 miles off the coast of Odesa, defying an order by a Russian warship to surrender with a memorable burst of profanity that became a rallying cry, immortalized on stamps and on billboards around the country.

In the video posted on Saturday, Mr. Zelensky honored the “heroes” who fought for Snake Island, calling the battle that ultimately forced Russian troops to withdraw last June “one of the most important” since the full-scale invasion.

“Although this is a small piece of land in the middle of our Black Sea, it is a great proof that Ukraine will regain every bit of its territory,” Mr. Zelensky said in the video, which showed him clambering off a boat and across a rocky landscape to lay blue and yellow flowers at a memorial.

It was not immediately clear when the video was filmed: The Ukrainian leader has been on a tour of NATO countries this week to drum up support for his country’s bid to join the alliance ahead of a summit next week.

The war has reshaped Ukraine’s relationship with the world, adding momentum to its bid to join NATO and turning Mr. Zelensky into a diplomatic juggernaut. He has used the global attention to help Ukraine push for billions in military aid to fend off Russian invaders, and recently embarking on a widely anticipated counteroffensive to take back occupied territory — a campaign that is under intense scrutiny.

Kyiv views membership in NATO as the ultimate guarantee of its security; its application in September to join the alliance was made against the backdrop of Russia’s full-scale invasion.

While Mr. Zelensky has acknowledged that Ukraine won’t be joining NATO anytime soon, given that such a move would force the mutual-defense alliance into direct military conflict with Russia, he has repeatedly urged its members to set out a timetable for accession. In recent months, he has expressed hopes that next week’s summit in Vilnius, Lithuania, could provide clarity.

With days to go before the meeting, Mr. Zelensky set out on a diplomatic offensive to press his case. He traveled to Bulgaria and the Czech Republic on Thursday and then Slovakia and Turkey on Friday, where he met with President Recep Tayyip Erdogan.

In a televised news conference early Saturday morning in Istanbul after his meeting with Mr. Zelensky, Mr. Erdogan said that “Ukraine deserves NATO membership with no doubt.”

But President Biden, who is scheduled to attend the summit during a trip to Europe next week, told CNN in an interview set to be broadcast on Sunday that Ukraine’s acceptance into NATO would most likely have to wait until after the war.

“I don’t think there is unanimity in NATO about whether or not to bring Ukraine into the NATO family now, at this moment, in the middle of a war,” Mr. Biden said, according to an excerpt published by CNN.

At the same time, Mr. Biden defended what he called the “very difficult” decision to provide Ukraine with cluster munitions, which are outlawed by many of America’s closest allies and which have been known to cause grievous injuries months or even years after fighting ends.

Ultimately, the president determined that depriving Ukraine of the weapons would amount to leaving it defenseless against Russia. He said it was a temporary move to hold Ukraine over until the production of conventional artillery rounds could be ramped up.

“The Ukrainians are running out of ammunition,” Mr. Biden said in an interview with CNN.

Ukrainian forces are about a month into their counteroffensive, a slow and bloody attack aimed at driving Russian forces from the country’s south and east.

Though bolstered by training and sophisticated new weapons from Western allies, Kyiv’s forces have only notched small gains, and the fierce fighting has cost Ukraine an undisclosed number of casualties, along with some of its newest tanks and armored vehicles.

While the counteroffensive rages, Russian forces have continued to fire missiles and launch drones on Ukrainian towns and villages far from the front lines.

On Saturday morning, Ukraine’s Air Force said that it had intercepted five Iranian-made attack drones overnight — but that others had evaded air defenses and hit “industrial and infrastructure objects” in two regions of the country.

One man was injured when drones hit the central Ukrainian city of Kryvyi Rih, according to the regional military administration. It said in a statement on the Telegram messaging app that an agricultural business had been hit, setting off fires that also destroyed equipment and several warehouses.

Local officials also reported artillery strikes in the Kharkiv region of northeastern Ukraine and the Kherson region in the south, which has come under relentless Russian bombardment.

On the eve of the 500th day, the United Nations said that it had confirmed the deaths of more than 9,000 civilians — including more than 500 children — since the full-scale invasion, calling it a “grim milestone” in a war that “continues to exact a horrific toll.” It warned that the true number of dead was likely to be much higher.