Syria's President Bashar al-Assad made a rare public visit on Wednesday to an electrical plant in centralDamascusto mark Labour Day, the presidency's official Facebook page said.

"President Assad is now visiting the Umayyad electrical plant inTishreenGardeninDamascusand congratulates its workers andSyria's workers on their holiday," the Facebook page said.

It published a picture of the president addressing a crowd of workers, some of them wearing or holding hard hats.

"Terrorists and those who support them are trying to plunge the country into darkness," state media quoted Assad as telling the workers, "whether on the intellectual or social level or by depriving citizens of basic services, chiefly electricity."

He praisedSyria's workers, saying they had "proven again during the war targeting our country that they will always be the country's strength and that the attacks targeting infrastructure will not stop them from pursuing their national duty."Assad, whose government is battling an uprising now in its third year, rarely appears in public.

Meanwhile, Syria peace envoy Lakhdar Brahimi is on the verge of quitting amid growing frustration at deadlocked international efforts to end the worsening conflict, diplomats said Wednesday.

Brahimi is "itching to resign but being persuaded to hang on for a few more days," said one UN Security Council diplomat. "He has told everyone that he wants to leave, there is little hope that he will stay," an Arab diplomat at the United Nations told AFP.

Syria's opposition on Wednesday denounced what it called "threats" by the head of Hezbollah, and warned against any intervention by the Lebanese Shiite group or by Iran in the Syrian conflict.

The speech was also criticised by Lebanese opposition leader Saad Hariri, who accused Hezbollah of "leading Lebanon to ruin" by intervening in Syria.

"The Syrians and the Lebanese hoped... that the Hezbollah leadership would stop their attacks on Homs and Damascus and take into account the gravity of the situation in the region," the Syrian National Coalition said in a statement.

"But they heard nothing but threats... and warnings against setting the region on fire and an admission of their interference in Syrian affairs," the key opposition movement said.

On Tuesday, Hezbollah chief Hassan Nasrallah acknowledged that members of the group are fighting inside Syria and suggested Iran and other states could intervene to support the Damascus regime against rebels.

President Barack Obama Tuesday warned against a rush to judgment on chemical weapons in Syria, but said proof of their use would trigger a "rethink" of his reluctance to use military force.

As critics complain that he let Syria cross a US "red line," Obama said Washington believed chemical weapons had been used in the country's vicious civil war but did not know exactly who had fired them.

At a White House news conference, Obama also appeared to set the criteria for a US military intervention as established proof that President Bashar al-Assad's regime directly ordered the use of chemical weapons.

As more Guantanamo Bay inmates join a hunger strike, Obama also pledged to have another go at closing what he described as the "not sustainable" war-on-terror prison and blamed Congress for the deteriorating situation.