Officials drove to Suu Kyi's lakeside Yangon home on Tuesday to read out an extension order in person, but it was unclear whether the extension was for six months or a year.
U.N. Secretary-General Ban Ki-moon, who just returned to New York from a weeklong aid mission in Myanmar, expressed disappointment but refrained from sharp criticism.
"The sooner restrictions on Aung San Suu Kyi and other political figures are lifted, the sooner Myanmar will be able to move toward ... restoration of democracy and full respect for human rights," he said.
He added that his special envoy for Myanmar, Ibrahim Gambari, would raise the issue of Suu Kyi with the junta.
Western nations were more forthright. U.S. President George W. Bush said he was "deeply troubled" by the extension and called for the more than 1,000 political prisoners in Myanmar to be freed. However, the State Department said it would not affect U.S. cyclone aid.
The 62-year-old Suu Kyi's National League for Democracy (NLD) won a 1990 poll by a landslide only to be denied power by the military, which has ruled the impoverished country for 46 years.
EU External Relations Commissioner Benita Ferrero-Waldner said "a historic opportunity was missed to give a sign of reconciling political life in Myanmar at a time when national and social cohesion, and solidarity and dialogue are more needed than ever."
Few had expected Suu Kyi to be released, but the extension was a reminder of the ruling military's refusal to make any concessions on the domestic political front despite its grudging acceptance of foreign help after the May 2 cyclone.
Hours before the extension, police arrested 20 NLD members trying to march to Suu Kyi's home.
Three weeks after the cyclone's 120 mph (190 kph) winds and sea surge devastated the delta, the United Nations said it had raised roughly 60 percent of its initial $200 million target for aid for Myanmar and aid workers were getting more access.
"We've reached just over a million people with some kind of aid," U.N. humanitarian affairs chief John Holmes told reporters.
Junta supremo Senior General Than Shwe promised U.N. chief Ban last week that he would allow all legitimate foreign aid workers access to victims across the country.
Holmes said he did not know if all roadblocks had been removed, but the situation was better.
"There's still a lot of people out there who have received nothing or certainly not enough," he said.
In the delta, thousands of beggars line the roads, and droves of children shout "Just throw something!" at passing vehicles.
Witnesses say many villages have received no outside help, and the waterways of the former Burma's "rice bowl" remain littered with bloated and rotting animal carcasses and corpses.
Much of the blame for the aid delay rests with the junta, which has been reluctant to admit a large-scale international relief effort for fear that would loosen the grip on power the army has held since a 1962 coup.
Nonetheless, diplomats and aid agencies see some signs of a shift in the stance of the reclusive junta.
State-controlled media on Tuesday praised U.N. agencies for taking prompt action to provide relief supplies after the cyclone, which left 134,000 people dead or missing.