The military chief of the main umbrella group of Syrian rebels, the Free Syrian Army, has accused Hezbollah fighters of "invading" Syria.
Gen Selim Idriss claimed that more than 7,000 fighters of the Lebanese Shia movement were taking part in attacks on the rebel-held town of Qusair.
Gen Idriss made an emotional appeal to Western powers on the BBC World Service's Newshour programme, calling for help, and appealed for more weapons "to defend citizens".
He said the FSA was fielding fewer than 1,500 fighters in the fight for Qusair, armed only with light weapons. More than 50,000 residents were trapped in the town and a "massacre" would occur if it fell, he added.
He also said he had received information that Iranian fighters were involved in the offensive.
Hezbollah fighters have been in Syria for some time now but their numbers appear to have grown rapidly over the last few weeks because of the intense fighting for control over the key town of Qusair.
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Gen Idriss also demanded that the Syrian opposition in exile stop internal wrangling and agree immediately to attend a peace conference scheduled for next month in Geneva.
The US State Department has demanded that Hezbollah withdraw its fighters from Syria immediately.
US State Department spokeswoman Jen Psaki said their presence was an "extremely dangerous" escalation.
The statements follow a meeting of the UN Human Rights Council in Geneva, which condemned government attacks on Qusair and the involvement of foreign fighters.