The United States blasted the commander of the UN peacekeeping force in Lebanon on Friday, accusing him of turning a blind eye to Hezbollah weapons smuggling.

US ambassador Nikki Haley said the 10,500-strong UNIFIL force was "not doing its job effectively" and singled out its Irish leader, Major General Michael Beary.

"What I find totally baffling is the view of the UNIFIL commander General Beary," Haley told reporters, accusing him of ignoring Hezbollah's arms dumps. 

"He seems to be the only person in south Lebanon who is blind. That's an embarrassing lack of understanding on what's going on around him," she said.

Asked about Haley's sharp criticism, United Nations spokesman Stephane Dujarric said of Beary "we have full confidence in his work."

Haley was speaking at UN headquarters as member states debate the future of UNIFIL, which is deployed to keep the peace on Lebanon's southern border with Israel.

The existing mandate, last modified in 2006, expires at the end of the month, and the United States would like to see its language toughened.

Washington wants the UN force to take a tougher line on Hezbollah, an Iranian-backed Shiite armed movement that is represented in Lebanon's government. Israel alleges that Hezbollah is restocking its arms dumps and missile batteries in southern Lebanon, under the eyes of Blue Helmet peacekeepers. But Russia, which is allied with Iran and thus with Hezbollah in support of regime forces in the conflict in neighboring Syria, wields a UN Security Council veto.  And US allies France and Italy, which have hundreds of soldiers in the UN force that would be in danger if it clashed with the militia, are also concerned. "Since 2006 there has been a massive flow of illegal weapons to Hezbollah, mostly smuggled in by Iran," Haley alleged. "They openly threaten Israel. Hezbollah is a terrorist organization that is very destabilizing to the region." Haley said the mandate obliges UNIFIL to work with the Lebanese Armed Forces to disarm illegal groups and that she would seek to underline this in the renewal.