Yemen's acting leader has threatened to leave office unless outgoing President Ali Abdullah Saleh and his allies stop interfering with his duties, a senior member of the opposition said Saturday.

A spokesman for Saleh's office denied reports of a rift between Saleh and his deputy, Abd-Rabbu Mansour Hadi, which looked like the latest obstacle to a Gulf-brokered plan to end months of protests and political deadlock that have paralysed the impoverished Arabian state.

Relations between Saleh and his deputy have deteriorated... and Hadi has informed Western mediators he will leave Sanaa if the interference in his jurisdiction continues, a senior member of the opposition Joint Meeting Parties (JMP) told Reuters on condition of anonymity.

A plan by Yemen's wealthier Gulf neighbours to transfer power has divided cabinet posts between the JMP and Saleh's General People's Congress (GPC), forming a national unity government intended to steer Yemen toward a presidential election in February under Hadi's leadership.

The United States and Saudi Arabia are keen for the plan to work, fearing that a power vacuum in Yemen is giving militants space to thrive alongside a key shipping strait, the Red Sea.

Saleh signed the deal in November, having backed out of it three times before, but question marks remain over the intentions of the veteran leader, who earlier this week said he would stay in Yemen, reversing a pledge to travel to the United States.

The unnamed JMP member said Hadi had warned mediators he would quit his position if Saleh and his followers continued to obstruct him.

Relations between Saleh and his deputy soured after Hadi refused the president's orders to reinstate his allies, and a spat in which Saleh supporters heavily criticised Hadi, leading the deputy president to boycott a subsequent meeting.

But the spokesman for the president's office told Reuters there was no truth to the report.

Let me be clear, there is no ongoing rift between the President and his principal deputy the Vice President, he said in a statement sent by e-mail.

MILITANT FIGHTING FLARES

More than a month after the Gulf deal was clinched, Yemenis angry at the offer of legal immunity to Saleh over the killing of demonstrators are still taking to the streets, calling for him to be put on trial.

United Nations High Commissioner for Human Rights Navi Pillay said Friday any guarantee of immunity to Saleh would violate international law, undermining the Gulf initiative.

In the south of the country, thousands of protesters marched almost 60 km (37 miles) from the port of Aden to their old homes in the city of Zinjibar but were blocked from completing their journey by militants who told them the area was mined.

We will not despair. We will try a second time and a third time and a fourth until we can enter and go back to our houses and our city. The militants must understand they are not wanted, said one marcher, Saleh al-Mosalli.

The marchers had been driven from the city in fighting between the army and Islamist fighters suspected of links to al Qaeda.

Underlining the lack of security, four militants and one soldier were killed in Zinjibar Saturday during fighting for control of the city, a local official said.

(Additional reporting by Mohammed Mukhashaf in Aden; Writing by Isabel Coles; Editing by Rosalind Russell)