Myanmar President Thein Sein declared a state of emergency in the Kokang region in the east and imposed a three-month period of martial law there in an announcement on state television on Tuesday night.
Fighting broke out on Feb. 9 between theMyanmar army and an ethnic Kokang force called the Myanmar National Democratic Alliance Army (MNDAA).
At least 47 Myanmar soldiers and 26 MNDAA fighters have been killed since then, the state-backed Global New Light of Myanmar newspaper reported, and thousands of civilians have fled, either to other areas in Myanmar or over the border into China.
On Tuesday, unknown attackers shot and wounded two people in a convoy of eight vehicles marked with the emblem of the Myanmar Red Cross Society that was attempting to transport civilians displaced by fighting in Laukkai, on the Chinese border, a witness said.
"We haven't had such an attack before," said Red Cross spokeswoman Shwe Cin Myint. "This would be the very first."
A government soldier blamed the attack on the MNDAA.
The clashes have alarmed China, which fears that the influx of Kokang refugees will swell, and called this week for peace on the border.
In 2009, fighting between the rebels and the army pushed tens of thousands of refugees into southwestern China.
The United States, which has been an enthusiastic supporter of reforms in Myanmar and efforts to bring about peace between the government and ethnic insurgent armies, said it was "deeply concerned" over the continued fighting and particularly by reports of the shooting of two Red Cross workers.
"We appeal to all sides to exercise restraint and return to dialogue," a State Department spokesperson said in an email. "Fighting will undermine the ongoing national reconciliation process."
The spokesperson said the United States was continuing to urge national and local authorities to permit immediate and unfettered access by humanitarian workers "to provide life-saving assistance to populations in need."
The MNDAA was formerly part of the Communist Party of Burma, a powerful Chinese-backed guerrilla force that battled the Myanmar government before splintering in 1989.
