Live Updates: Even as It Mounts Its Own Offensive, Ukraine Braces for a Russian Attack in the South

President Volodymyr Zelensky, responding on Tuesday to a citizen petition that called for the legalization of same-sex marriage in Ukraine, raised the prospect of “civil partnerships,” but said the Constitution’s definition of marriage as the union of one man and one woman could not be changed during wartime.

Mr. Zelensky was responding after the petition garnered support from more than 25,000 Ukrainians. Ukraine does not recognize marriage rights for same-sex couples, nor does it have a statute allowing them to enter into civil unions. Calls to grant those couples equal rights have grown in part because of the sacrifices of L.G.B.T.Q. soldiers helping the country fight Russia’s brutal invasion.

Under Ukrainian Ministry of Defense regulations, the military must inform the parents and spouse or other close relatives of a soldier who is killed. But those rules do not apply to same-sex couples who are unable to legally wed.

In Ukraine, same-sex couples do not have the automatic right to visit a hospitalized partner, to share property ownership, to care for a deceased partner’s children, to claim the body of a partner killed in war or to collect death benefits from the state.

The petition urging Mr. Zelensky to support marriage rights for same-sex couples was initiated by Anastasia Sovenko, 24, an English teacher from Zaporizhzhia, in southern Ukraine, who identifies as bisexual.

She had said that after reading an article about heterosexual soldiers rushing to marry their partners before heading to war, she felt sad, angry and frustrated that L.G.B.T.Q. soldiers did not have that option.

Mr. Zelensky, in his response, wrote that democratic societies are measured in part by the way they defend equal rights for all citizens and that “each citizen is an indivisible part of civil society, for whom all rights and freedoms fixed in the Constitution of Ukraine extend.”

“All people are free and equal in their dignity and rights,” he wrote.

Mr. Zelensky noted, however, that the Constitution of Ukraine states that “marriage is based on the free consent of a woman and a man,” and said the document could not be changed during a state of martial law, which he declared in February, after Russia invaded.

At the same time, Mr. Zelensky wrote, the government had “worked out options for a solution regarding the legalization in Ukraine of registered civil partnerships under the auspices of work on the confirmation and protection of human rights and freedoms.”

He added that he had asked Ukraine’s prime minister, Denys Shmyhal, to review the issue raised by the petition and to report back with his findings.

“The response is more positive than it could have been,” Inna Sovsun, an opposition lawmaker in the liberal Holos party, which favors L.G.B.T.Q. rights, wrote on Facebook. “But at the same time — not clear enough.”

Because amendments to the Constitution are now impossible, she wrote, civil partnerships would be an “acceptable interim alternative.”

“However, where are the options that the President is talking about?” Ms. Sovsun wrote. “Why are they not presented for discussion and not submitted to Parliament?”

Ms. Sovenko, the petition’s author, said she was happy that Mr. Zelensky had mentioned civil partnerships for same-sex couples.

“That’s the beginning,” she wrote in an email. She said she was “full of hope” that, after the war, the government would give same-sex couples an opportunity to get married.

“I’m proud the response wasn’t all negative,” she said, “and I’m really happy this happened.”

The drive for same-sex marriage faces significant resistance in a country where the Eastern Orthodox religion and traditional sexual mores are deeply embedded in the social fabric. Opponents include some conservative members of Mr. Zelensky’s own party, who have called for a law fining “homosexual propaganda.”

But gay rights advocates in Ukraine hope that Mr. Zelensky will eventually support same-sex marriage rights, helping the country burnish its liberal credentials as it seeks to join the European Union and draw closer to the West.

Maham Javaid and Dan Bilefsky contributed reporting. David Kurkovskiy contributed a translation.