YANGON (AFP) - Myanmar's president has vowed "not to lose an inch" of territory in clashes with ethnic rebels in a region bordering China, state-backed media said Tuesday, after intense fighting sent tens of thousands fleeing across the frontier.

Violence between Myanmar's army and Kokang rebels in northeastern Shan state has sparked alarm in China, which has warned of a threat to border security after some 30,000 reportedly crossed into Yunnan province in the last week.

Myanmar's President Thein Sein said the military was "protecting sovereignty and ensuring territorial integrity", the Global New Light of Myanmar reported, after some of the most fierce recent clashes in a nation plagued by ethnic minority insurgencies.

The former junta general "vowed not to lose an inch of Myanmar's territory" during a visit to wounded soldiers in hospital, the report said.

The Myanmar military has launched airstrikes against rebels who tried to capture Kokang's main town of Laukkai, where dozens have been killed on both sides in raging street battles.

On Monday, Chinese foreign ministry spokeswoman Hua Chunying said in Beijing that more fighting "will have an impact on the stability of the China-Myanmar border areas and the security on the Chinese side of the border".

Most displaced local people, who are largely ethnic Chinese, have fled across the border into China.

Beijing's official Xinhua news agency, quoting the press office of Yunnan's Lincang City, said more than 30,000 have crossed the border since fighting began on February 9, adding that Chinese officials had provided them with food and medicine.

Some 2,000 have also headed to central Myanmar, according to the country's state media.

Myanmar authorities have blamed local Kokang rebel leader Phone Kya Shin for the fighting and called on Beijing to rein in any local officials who might be helping the group on its side of the border.

Hua sought to reassure China's southern neighbour, saying Beijing would not allow any group to "carry out activities undermining China-Myanmar relations... (from) within Chinese territory" and urged restraint on both sides of the conflict.

The Kokang region has been relatively calm since 2009, when a huge assault by Myanmar's army against the Kokang rebels saw tens of thousands of people flood over the border into China.

The fighting earned Myanmar's then-junta a rare rebuke from China, which at the time was almost its sole ally on the international stage.