is preparing for a possible war with Israel to relieve perceived Western pressure to topple Syrian President Bashar Assad,
its guardian ally, sources close to the movement say.
The radical Shi'ite group is watching the unrest in neighboring Syria with alarm and is determined to prevent the West from exploiting popular protests to bring down Assad.
Hezbollah supported pro-democracy movements that toppled Western-backed leaders in Tunisia and Egypt, but officials say it will not stand idly by as international pressure mounts on Assad to yield to protesters.
It is committed to do whatever it takes politically to help deflect what it sees as a foreign campaign against Damascus, but it is also readying for a possible war with Israel if Assad is weakened.
"Hezbollah will never intervene in Syria. This is an internal issue for President Bashar to tackle. But when it sees the West gearing up to bring him down, it will not just watch," a Lebanese official close to the group's told Reuters.
"This is a battle for existence for the group and it is time to return the favor (of Syria's support). It will do that by fending off some of the international pressure," he added.
Hezbollah and Syria have both denied that the group has sent fighters to support a military crackdown on the wave of protests against Assad's rule.
Hezbollah believes the West is working to reshape the Middle East by replacing Assad with a ruler friendly to Israel and hostile to itself.
Analysts rule out the possibility of a full-scale regional war involving Syria, Iran and Lebanon on one side against Israel backed by the United States. A war pitting Hezbollah against Israel was more likely, they said.
"There might be limited wars here or there but nobody has the interest (in a regional war)," said Lebanese analyst Oussama Safa. "The region is of course heading towards radical change... How it will be arranged and where it will lead is not clear."
Western intelligence sources say the movement's arsenal has been more than replenished since the fighting ended, with European-led UN peacekeepers in southern Lebanon powerless to prevent supplies entering mostly from Syria.
In the meantime Hezbollah, which has praised other Arab uprisings and enjoys strong support among ordinary Arabs over its confrontations with Israel, has seen its image tarnished because of its support for Assad.
"The events in Syria have not impacted Hezbollah in a significant strategic sense, but have certainly put the party in an uncomfortable position," said Elias Muhanna, a Middle East scholar at Harvard.
"The fact that Nasrallah has supported the regime's war against the opposition in Syria while attacking similar regime actions in Tunisia, Libya, Egypt, Bahrain, and Yemen has been pointed out by many as a blatant double standard."
Hezbollah argues there is no contradiction in its position, saying Assad has popular support and is committed to reform.
"When the regime is against Israel and is committed to reforms then Hezbollah decision is to be by the side of the people and the leadership through urging them for dialogue and partnership," the Lebanese official said.