(CNN) -- Hezbollah forces are fighting on the side of the Syrian government in the strategic town of Qusayr against rebels, its leader said Saturday.
"Syria is the backbone of the resistance (in the region) and its main supporter," Hezbollah Secretary-General Hassan Nasrallah said in a speech broadcast live on Hezbollah-owned Al Manar Television. "The resistance will never stand by while its backbone is exposed."
The pro-regime and pro-Iranian Shiite militia, based in Lebanon, is regarded as a terrorist group by the U.S. government and Sunni countries.
Nasrallah's 75-minute speech marked the first time he has acknowledged Hezbollah forces are fighting in the conflict.
It came shortly after an opposition group said that shelling by Hezbollah fighters had killed at least 30 people in the strategically important western Syrian town of Qusayr.
The Local Coordination Committees of Syria said the fatalities included women and children. Another opposition group, the Syrian Observatory for Human Rights, said that 27 of the dead were rebel fighters.
The Free Syrian Army said it had killed 45 members of Hezbollah.
In eastern Lebanon's Bekaa Valley, thousands of Hezbollah supporters watched the speech on a big screen. "Regarding this battle, like all the battles before it, our men are there and we will make victory there," he said to cheers.
Nashrallah accused the rebel forces of belonging to extremist religious movements, and said Hezbollah forces were acting to protect Lebanon.
"If the armed groups control Syria or specific Syrian provinces, especially those on the Lebanese border, then we consider them a great threat to Lebanon, the unity of the nation and all Lebanese, not just Hezbollah or Shiites in Lebanon."
Hezbollah fighters represent only a tiny minority of foreign fighters in Syria, Nasrallah said.
"Tens of thousands of (rebel foreign) fighters did not bother the so-called Friends of Syria countries whose representatives met in Amman a couple of days ago," he said, referring to the opposition umbrella group, which met in Jordan.
"But the interference of a small group from Lebanese Hezbollah was considered a foreign intervention. We never interfered (in Syria) until a few months ago, so I am completely honest with you."
Nasrallah said the government in Damascus has always been willing to negotiate a peaceful resolution with rebels, but that the opposition -- hopeful that President Bashar al-Assad would be ousted -- has refused to do so.
For its part, the government-run Syrian Arab News Agency said government forces were carrying out an operation against "armed terrorist groups" in the town, and had killed large numbers of them.
In Istanbul, Turkey, a Syrian opposition leader called for weapons, fighters and other aid.
"In today's Syria, terrorism is killing tolerance; ignorance is killing logic; barbarity is overwhelming civility," Syrian National Coalition Acting Chairman George Sabra told reporters.
Referring to Hezbollah forces, he said, "You are heading in the wrong direction. The resistance should not be in the north, in Qusayr or Daraya. Due to sectarianism and blindly following the most tyrant regime in the world, some Lebanese are sent to Syria."