The main political opposition coalition, impatient with President Ali Abdullah Saleh's repeated earlier refusals to sign the Gulf-brokered pact, expressed scepticism about the presidential decree, which called for yet more talks before any signing.

Nevertheless, the decree "irrevocably" empowered Vice President Abbd-Rabbu Mansour Hadi to sign the Gulf Cooperation Council (GCC) plan to transfer power from Saleh.

"(The vice president) is given the necessary constitutional authorities to conduct a dialogue with signatories of the GCC initiative and agree on a lasting mechanism to implement and sign it on (Saleh's) behalf under regional and international auspices, resulting in early presidential elections," it said.

Saleh, 69, has been recuperating in neighbouring Saudi Arabia since he was severely wounded in a bomb attack in June. He had backed out of signing the transition deal three times before the assassination attempt inside his presidential compound.

Months of fruitless negotiating had frustrated tens of thousands of anti-Saleh protesters as daily demonstrations and sporadic clashes paralysed the impoverished Arab state.

The United States and oil giant Saudi Arabia, both targets of foiled attacks by al Qaeda's Yemen-based wing, have pushed for the GCC deal for months, wary that growing turmoil is giving more room for Islamist militants to operate.

Hours after the decree was issued, motorcycle-riding masked gunmen shot dead a son of opposition leader Mohammed Ashal from the Islamist party Islah in Sanaa, a family member said.

The killing followed the assassination on Sunday of an army brigadier general in the capital, and an attempt to kill a colonel who led troops fighting al Qaeda-linked militants in the south.

A Yemeni analyst said Saleh's authorisation to Hadi appeared sufficient to get the GCC plan enacted.

"This is enough to ensure that the Gulf initiative is implemented and that early presidential elections are held, and will safeguard Yemen from armed conflict," said Ali Seif Hassan.

Room for Doubt

But the opposition coalition, the Joint Meeting Parties (JMP), appeared wary of the conditions set by Saleh's decree.

"Any talk of a dialogue before signing the Gulf initiative is either an attempt at wasting time or deceiving public opinion," said JMP spokesman Mohammed Qahtan.

The coalition vowed that mass protests would continue.

"The JMP calls on the young men and women of the revolution to continue escalating the peaceful revolution until the whole regime falls," it said in a statement.

Violence is unsettling many parts of Yemen.

Air raids and fighting shook the flashpoint town of Arhab, just 40 km (25 miles) north of Sanaa, where tribesmen have thrown their weight behind the protesters and have clashed frequently with troops who have a military base there.

Four armed tribesmen were killed while trying to block an advance by the Republican Guard, opposition sources said.

Saleh's formal departure may still be months away even if the transition agreement goes through.

Under modifications recently agreed under a U.N.-mediated "operational mechanism", he would transfer powers to his vice president, after which presidential elections must be held within three months.

But as Saleh has only transferred authority for negotiating the transition deal, it appears that the three-month period will only kick off once Hadi signs the GCC plan in his stead.

If the deal is signed, a newly elected president, expected to be Hadi, would head a two-year interim unity government that would draft a new constitution and negotiate with Shi'ite Muslim rebels in the north and armed separatists in the south.