The Myanmar government on Thursday called for an immediate ceasefire between its troops and rebels from the Kachin ethnic group whose headquarters have been under air assault since last week.
"Both sides should meet face to face and stop fighting," said Major Zaw Htay, director of the President's Office in the capital, Naypyitaw.
Htay said the government had contacted the Kachin Independence Organization and its armed wing, the Kachin Independence Army (KIA), to discuss a ceasefire, but the insurgents had "not replied yet."
UN Secretary General Ban Ki-moon on Wednesday condemned the military offensive against the Kachin.
Myanmar authorities should "desist from any action that could endanger the lives of civilians living in the area or further intensify the conflict" in the region, he said.
An estimated 65,000 Kachin civilians have been displaced by fighting in the northern state.
The army has been fighting the rebels, who have been seeking greater autonomy for their region for more than 50 years, since a long-standing ceasefire broke down in June 2011.
On December 28, the military launched an air assault on the KIA's headquarters in Laiza on border between Kachin State and China, 820 kilometres north-east of Yangon.
KIA sources claimed the military used Chinese-made jet fighters and helicopter gunships to attack Laiza, dropping small bombs and some "unknown chemicals" on the base.
"More than 40 KIA soldiers have been hospitalized after breathing the gas from the chemicals," said Nawdin Laphai, editor of the Kachin News Group, a pro-KIA publication.
Htay denied that fighter jets had been used in the attack.
"The military used trainer aircraft at the KIA areas, and we can say that we did not start the fighting," Htay said.
The rebels have inflicted heavy casualties on the army over the past 18 months, a possible motivation for the military to ignore Naypyitaw's orders to negotiate a ceasefire, US-based academic and Myanmar watcher Win Min has said.
According to the state-run New Light of Myanmar, "the KIA blasted rail trucks over 50 times and sabotaged machinery for construction of road and bridge over 100 times" in that period.
"The government is making strenuous efforts for ensuring regional security, peace and stability and development in Kachin State, but the KIA group still pursues destructive acts against regional development and security," the state-run daily said Thursday.