Hassan Nasrallah, the secretary-general of the Lebanese Hezbollah group, has said his movement would never recognise Israel, rejecting a US precondition for dialogue with the group it considers a terrorist organisation.
"To those who impose conditions on us, we say: We will never recognise Israel," Nasrallah said in a speech on Friday.
The White House said on Tuesday that both Palestinian movement Hamas and Hezbollah must renounce violence and recognise Israel before they can expect even low-level US engagement.
"We reject the American conditions ... Today, tomorrow and after 1,000 years and even until the end of time, as long as Hezbollah exists, it will never recognise Israel," Nasrallah said.
A senior US official said on Thursday he was unhappy with a British decision to open low-level contact with Hezbollah and suggested London only indirectly informed the new US administration ahead of time.
Nasrallah also saluted recent moves to smooth over Arab differences, with Saudi Arabia and Egypt seeking to improve ties with Syria, which has supported Hezbollah.
"All Arab reconciliation reinforces us," he said.
He called for Riyadh and Cairo to "extend a hand" to Iran, Hezbollah's main backer.
A Hezbollah-led alliance has veto power over major decisions in the current unity government formed in July following a political crisis that brought Lebanon to the brink of civil war.
Legislative elections are set for June 7 in Lebanon