AFP - The Malaysian and Thai prime ministers made a historic joint visit Wednesday to Thailand's restive south where a bloody separatist insurgency has left more than 4,000 dead.

Najib Razak and his Thai counterpart Abhisit Vejjajiva officially named a "Friendship Bridge" spanning their shared border and were to visit an Islamic school in Thailand's troubled Muslim-majority southern provinces.

Security was tight for the trip after a spike in deadly bombings and shootings that has left 10 people dead since Monday, underscoring the difficulties in finding a solution to the six-year uprising.

"I have no doubt in my mind that this Friendship Bridge will serve its noble purpose and boost further the various economic, social and cultural activities between our two countries," Najib said at the naming ceremony.

The two leaders arrived at the bridge near the Thai town of Bukit Ta on military helicopters. Thai army choppers circled the area and authorities cut mobile phone signals to guard against possible remote-controlled bombs.

Hundreds of Thai troops manned checkpoints leading to the border and bomb disposal squads checked the area, while police boats patrolled the muddy waters of the Sunai Kolok River flowing below the bridge.

In the latest violence, one forensic policeman was killed and three security officials wounded in twin bomb blasts by a river bank in Yala's provincial town on Wednesday morning.

Earlier Wednesday, two Thai marines were wounded in a bomb and gun attack in Narathiwat province, where the bridge is located, after the troops tried to collect banners criticising Abhisit's policies on the south.

Another roadside bomb wounded two troops and a policeman protecting teachers in neighbouring Yala province, while on Tuesday a bomb killed a soldier in a border town.

A soldier wounded in a clash with insurgents late Tuesday on a road due to be used by Abhisit and Najib died in hospital overnight, taking the toll from the incident to three security officials and one militant.

Two people were killed by a powerful motorcycle bomb in the town of Narathiwat on Monday while two other civilians were shot dead earlier this week.

The two-lane bridge was opened in December 2007 by Abdullah Ahmad Badawi and Surayud Chulanont, the then-prime ministers of Malaysia and Thailand, in a bid to boost the border region's economy.

Badawi did not visit any other parts of southern Thailand at the time.

In Bangkok on Tuesday, Najib pledged his support for Thailand's "constructive" plans to curb the violence after holding talks with Abhisit but said it remained essentially a domestic problem for Thailand.

Abhisit said Najib had also offered support "in terms of making sure that the people engaging in violence do not use Malaysia as a base for their operations".

Thailand has in the past accused Malaysia of failing to prevent insurgents criss-crossing the porous 650-kilometre (400-mile) border.

But since Najib came to power in April the rhetoric has softened. In October he called on Thailand to offer "some form" of autonomy to the region, a proposal backed by Abhisit, who called it the "right approach".

The two leaders were later Wednesday due to visit one public school and one Islamic school, a handicraft village and a "widow village" that shelters some 140 families affected by the unrest.

Tensions have simmered in the troubled region, formerly an autonomous Malay Muslim sultanate, since it was annexed in 1902 by mainly Buddhist Thailand. The current insurgency erupted in January 2004.

Brussels-based think-tank International Crisis Group said Tuesday that Thailand must deal with the perceived impunity of security forces and disarm civilian militias.