Great Britain, Australia and Canada joined the United States in advising their citizens about possible violent attacks in Myanmar.

An initial warning on Wednesday by the US Embassy in Yangon said Myanmar’s security forces are “investigating reports of potential attacks” in the country’s capital, Naypyitaw. It said the possibility of attacks extended to the coming months in Naypyitaw, Yangon and Mandalay, its three biggest cities.

However, there was no explanation of why there might be attacks on specific dates in September and October. There were no reports that any attack had taken place on Thursday.

The Canadian and British advisories specified that the potential attacks could be bombings. “The Myanmar government has not corroborated this information. No specific measures are recommended at this time,” the statement issued by Britain’s Foreign and Commonwealth Office said.

Several armed ethnic groups are battling the Myanmar government for more autonomy, but the fighting usually takes place in their home regions in border areas.