The warning in a joint statement to be made at the East Asia Summit comes amid reports that extremists from Malaysia and Indonesia may have formed a Malay-speaking military unit in Syria. Scores of Muslims from both countries are believed to have joined the Islamic State group this year.

The statement will stress that extremism, radicalism and terrorism must be addressed in a comprehensive manner through a "multifaceted approach" including addressing underlying factors, officials said.

The summit that brings together leaders from Asia and Europe will discuss highly contentious overlapping claims in the South China Sea, although officials say no breakthrough is expected.

China has been aggressively pursuing its claim to most of the sea through which a third of the world's shipping transits. The region is also believed to have oil and gas beneath its seabed. Other claimants are Vietnam, the Philippines, Taiwan, Brunei and Malaysia.

The Naypyitaw gathering, which includes US President Barack Obama, will discuss how Asia, which accounts for 60 per cent of the world's population, is acutely vulnerable to Ebola, the disease which has killed 5000 people in West African nations, officials said.

The gathering has focused world attention on Myanmar, a former pariah state also called Burma, which has come under renewed criticism for its treatment of Rohingya minority Muslims in western Rakhine state.

The country is also seen as having stalled a reform process that was hailed by Western nations two years ago.

US officials said Mr Obama will press Myanmar for more progress on constitutional reform ahead of elections in 2015 that would open the way for opposition leader Aung San Suu Kyi to run as president. Washington believes Myanmar's reforms have not gone far enough.

Diplomats said the meeting would highlight concerns about a retreat from free speech, human rights and democracy across south-east Asia.

In Thailand following a coup in May, military rulers continue to enforce blanket restrictions on the rights to freedom of expression and peaceful assembly. Gatherings of more than five people are banned as a centuries-old martial law remains in place across the country.

In Malaysia, authorities are increasingly using a draconian Sedition Act to silence peaceful critics with dozens of people facing charges. Popular opposition leader Anwar Ibrahim is facing years of jail following a relentless campaign of persecution against him and his family.

In Vietnam, authorities have targeted bloggers, throwing dozens into jail, while in neighbouring Cambodia the government of  Prime Minister Hun Sen is moving to criminalise and censor online expression, and security forces have shot dead protesting garment workers.

Amnesty International's research director for south-east Asia Rupert Abbott said the region has come a long way in recent decades in its efforts to improve respect for human rights, a change that has helped unleash some of the region's economic potential.

"Yet the restrictions on free speech sweeping across the region are troubling and are only likely to discourage much-needed investment overseas," he said.