Israel’s strike on Beirut on Tuesday came as senior leaders of the regional militant groups backed by Iran, known as the “axis of resistance,” were in Tehran for the inauguration ceremony of Iran’s new president, Masoud Pezeshkian.

Even before the Israeli attack targeting a Hezbollah commander, Iranian military leaders were expected to meet with the militant leaders on the looming threat of war between Israel and Hezbollah.

In a statement on Tuesday, Iran’s embassy in Beirut condemned “the cowardly and criminal attack,” according to Iranian media. In a separate statement, Iran’s foreign ministry spokesman, Nasser Kanaani, also strongly condemned the attack and said it was “a clear violation of Lebanon’s sovereignty.” He warned that Hezbollah and Lebanon had the right to retaliate against Israel and said that Iran would hold Israel and the U.S. responsible for a wider regional war.

The threat of a regional war could present Mr. Pezeshkian with the first major crisis of his presidency. Iran has maintained a dual policy of averting direct engagement in all-out war while supporting a network of militant groups that have opened fronts on Israel from Lebanon, Yemen, Iraq and Syria.

Inauguration ceremonies are typically an occasion for Iran to showcase its domestic politics to an audience of foreign dignitaries. But this year, the volatile state of the Middle East, and Iran’s key role in shaping events through its influence over a network of militant groups, were also prominent themes.

On Tuesday, Mr. Pezeshkian, a heart surgeon who has portrayed himself as a reformer and beat a hard-line conservative in a July runoff vote, placed his hand on the Quran to take the oath of office, standing at a podium decorated with green, white and red flowers — the colors of Iran’s flag. In his speech, he addressed Iran’s support for the Palestinian cause.