BEIRUT — Attacks against Iranian-allied forces in three countries, all blamed on Israel, escalated tensions across the Middle East on Sunday, drawing threats of retaliation and intensifying fears that a bigger conflict could erupt.

The attacks Saturday and Sunday targeted Iranian forces and their proxies in Lebanon, Syria and Iraq, in what appeared to be a significant escalation of Israeli efforts to contain the expansion of Iranian influence in the region that could jeopardize the continued presence of U.S. troops in Iraq and draw Lebanon into a new war.

All of the attacks were shrouded in the mystery that engulfs much of the shadowy conflict between Israel and Iran. Israel confirmed that it was responsible only for the first attack, in which its warplanes struck overnight Saturday on what military officials said was an Iranian-operated base in Syria preparing to launch a major drone attack against Israel. At least two members of the Lebanese Hezbollah militia were killed, Hezbollah said.

Israel's military claimed it struck on late Aug. 24 an Iranian-operated base inside Syria preparing to launch "killer drones" on Israel. (Reuters)

Hours later, a drone armed with explosives hit a building housing a Hezbollah media center in the southern suburbs of Beirut. Hezbollah accused Israel in what would be the first Israeli attack in Lebanon since the 2006 war and said it would respond by shooting down Israeli drones in Lebanon. Israel did not confirm or deny responsibility.

Later Sunday a commander with an Iranian-backed militia in Iraq was killed by a drone strike in the western Iraqi town of al-Qaim, compounding tensions raised by a recent series of strikes apparently conducted by Israel against Iranian-backed militias in Iraq.

Any one of these incidents would have marked an escalation in a steadily intensifying confrontation between Israel and the Iranian-backed militias that have expanded their reach across the Middle East in recent years. Taken together, they signal a “new era” in Israel’s attempt to roll back Iran’s presence in the region, said Hilal Khashan, a professor of political science at the American University of Beirut.

“The Israelis are telling everyone that they are expanding the scope of their attacks against Iran and its allies,” Khashan said. “The fact that these attacks occurred ushers in a new stage in the confrontation between Israel on the one hand and Iran and its allies on the other. Israel is showing it is determined to prevent Iran from expanding its influence in the Middle East.”

In an angry speech on Sunday night, Hezbollah leader Hasan Nasrallah called the drone strike on the media center “a very, very dangerous violation” and said Hezbollah would strike back from Lebanon in retaliation for that attack in Beirut and the deaths of Hezbollah members in Syria.

Hezbollah will also try to down any of the Israeli drones that routinely fly over Lebanon, he said. He warned Israelis living in northern Israel to brace for Hezbollah attacks on Israeli territory.

“This is a new phase imposed by the enemy, and we are up for it,” he said in the speech delivered by video to crowds of cheering Hezbollah supporters in eastern Lebanon’s Bekaa Valley. “What happened last night will not happen without response.”

The Israeli claims suggested Iran already might be contemplating retaliation for recent attacks on weapons storage facilities in Iraq.

The Israeli strikes overnight Saturday targeted the town of Aqraba, southeast of Damascus, where the Israeli Defense Forces said the Iranian Quds Force and allied militias were readying a “large-scale attack of multiple killer drones on Israel.”

Syrian state media said Syrian air defenses were activated to intercept the strike and brought down most of the Israeli missiles. Videos posted on social media showed a huge blaze raging and anti-air missiles streaking through the night sky.

Prime Minister Benjamin Netanyahu hailed what he called “a major operational effort.”

“Iran has no immunity anywhere,” he tweeted. “If someone rises up to kill you, kill him first.”

Israel has struck hundreds of targets in Syria over the last seven years, most of them Iranian forces or Iran’s efforts to deliver sophisticated weaponry to Hezbollah in Lebanon via Syria.

This is the first time, however, that Israel has accused Iran of attempting such a large-scale attack on Israeli territory. Had such an attack taken place, it could have triggered an all-out war.

It was impossible to confirm whether such an attack was planned. Iran’s Islamic Revolutionary Guard Corps, which commands the Quds Force, denied that any Iranian targets had been struck in Syria.

“This is a lie and not true,” Maj. Gen. Mohsen Rezaei said, according to the Iranian news agency ILNA. “Israel and the United States do not have the power to attack Iran’s various centers and our advisory centers have not been harmed.”

Israel placed its military on high alert against possible retaliation, a military spokesman said.

Over the past month, Israel has expanded its strikes to Iraq, hitting four Iranian-backed militia bases where weapons were being stored in what are suspected to be drone strikes. Officially, Israel has only hinted that it was responsible, but U.S. and Israeli officials have told news outlets that Israel did carry out the attacks, Israel’s first on Iraq since 1981.

The killing later Sunday of a commander with Kataeb Hezbollah, one of Iran’s staunchest allies in Iraq, was also a first. The drone strike in the western Iraqi town of al-Qaim was instantly blamed on Israel and prompted a call from one of the most powerful blocs in the Iraqi parliament for U.S. troops to withdraw from Iraq.

The killing “represents a dangerous turning point,” the Iran-allied Fatah coalition said in a statement. The coalition cited widespread suspicions in Iraq that the United States is aiding the Israeli attacks and said it believes “there is no need for the American presence.”

An Israeli spokesman did not confirm or deny responsibility. The U.S. military has responded to allegations of involvement by pointing out that it is in Iraq at the invitation of the Iraqi government and only to fight the Islamic State terrorist group.

Israel also declined to confirm or deny that it was behind the blast at a Hezbollah office in Beirut. Initial reports from Hezbollah suggested the drone had exploded accidentally in the air, but Nasrallah said in his speech that a drone armed with explosives had deliberately crashed into the building housing Hezbollah’s media office.

About an hour later, he said, a smaller drone hovered in the vicinity of the building and crashed after local residents threw stones. Hezbollah’s account could not be independently confirmed, but videos taken at the scene showed shattered windows and damage to furniture at the Hezbollah office.

Mustafa Salim in Baghdad and Suzan Haidamous in Beirut contributed to this report.