Two explosions went off in military-ruled Myanmar's main city today, three weeks before a controversial constitutional referendum.

Police and witnesses said there did not appear to be any casualties.

Witnesses said the first blast, at 8.20 p.m. (2.50 p.m. CET), occurred under a car parked on a street corner in downtown Yangon, the former capital. Security police arrived within minutes and sealed off the area.

A few minutes later, another small explosion was heard near the Traders Hotel, a posh city centre hotel favoured by foreign businessmen. Police refused to give any details.

It was not immediately clear what caused the blasts.

Myanmar's junta, the latest face of more than four decades of military rule in the former Burma, have warned about the possibility of "terrorist attacks" in the run-up to the May 10 referendum.

The opposition National League for Democracy and a host of underground anti-regime groups are campaigning for rejection of the charter, which gives the army 25 percent of seats in parliament and the right to suspend the constitution at will.

Previous blasts in the city have invariably been blamed on dissident pro-democracy groups or some of the ethnic militias still waging guerrilla wars for self-government or their own homeland in the former British colony.