By Tim Cocks

KAMPALA (Reuters) - Rwanda and Burundi officially joined the East African Community (EAC) on Monday, signing accession treaties that will expand the regional economic bloc to five nations and boost trade.

Officials said their entry into the EAC, alongside Kenya, Uganda and Tanzania, would be effective from July 1.

"I would like to welcome the two new members of the East African Community. We hope to make this combination of five countries a big success," said Kenya's president and outgoing EAC chairman Mwai Kibaki at the signing ceremony.

Rwanda and Burundi hope to benefit from an EAC customs union, which began setting common external tariffs for goods entering the region in January 2005.

The move would also allow the tiny central African neighbours to join a planned political federation, including a common market for the region's combined population of 110 million, a monetary union and a common president and parliament by 2010.

The heads of state and other delegates also discussed reconstituting the East African court of justice, progress on the customs union and the adoption of an EAC anthem.

Both struggling to emerge from years of civil war, Rwanda and Burundi were accepted as members last November.

"A united and prosperous East African Community will enable us to assert our... interests in the global context," Rwandan President Paul Kagame said shortly after signing, calling the move a milestone.

Rwanda is rebuilding an economy shattered after a 1994 genocide in which 800,000 people were butchered, while Burundi is still in talks with rebels to end an insurgency that has killed some 300,000 of its citizens since 1993.

Industry leaders in both countries fear they will be forced out of business by an influx of cheap, imported products from existing members, especially Kenya, whose manufacturers dominate the region's markets.

Burundia President Pierre Nkurunziza appealed for help in mitigating these effects.

"Burundi, which is emerging from crisis, remains the weak link in the chain," he said. "We require strong support from our partners to alleviate the burden of the customs union."

But the country's business leaders feared the worst.

"The long crisis ... has affected the productivity of our industries. They are incapable of resisting the strong competition from Kenya or Tanzania," said Cyrille Sigejeje, secretary-general of the Burundi Chamber of Commerce and Industry.