UN Secretary-General Ban Ki-moon expressed "grave concern", and called on the Burmese government not to undermine Burma's national reconciliation process, his spokeswoman said.
Ms Su Kyi's party, the National League for Democracy (NLD), which won a landslide election victory in 1990 only to be denied power by the military, "strongly condemned" the charges, which come two weeks before her latest detention was due to expire.
British Prime Minister Gordon Brown earlier said he was "deeply disturbed" by the charges and he accused the Burmese military government of seeking "any pretext, no matter how tenuous" to extend the detention.
The EU special envoy to Burma, Piero Fassino, said there was "no justification" for the detention.
Thailand's prime minister also expressed concern on behalf of the Association of South East Asian Nations (Asean), one of the few groups that allow Burma as a member.
"We would like to see positive steps being taken," Thai Prime Minister Abhisit Vejjajiva told Reuters, adding that the group was "concerned" by the recent events.
After visiting her at Rangoon's notorious Insein prison, Ms Suu Kyi's main lawyer, Kyi Win, told the BBC's Newshour programme that she was physically well and her spirit was strong.
Myint Swe, BBC Burmese Service
Aung San Suu Kyi's house arrest is due to expire at the end of May. There is a legal requirement to charge her or else release her from detention.
At this pivotal time, the incident of the US citizen allegedly staying at her compound is being seen by critics as a pretext to put her behind bars.
The charges against Ms Suu Kyi of breaching the terms of her house arrest show that the authorities will not tolerate any challenge to their power and legitimacy.
Despite international pressure and concern, the Burmese government seems intent on pursuing elections in 2010, which the generals think will legitimise their rule.
"From all appearances, she is quite well and of course she is a little thin, that's all," he said of the 63-year-old Nobel Peace laureate.
He said she asked him to tell her friends that she was physically well and even offered him encouragement, saying: "You have to have a very strong and stout heart".
Reports say Ms Suu Kyi was charged under the country's Law Safeguarding the State from the Dangers of Subversive Elements.
The charges carry a maximum jail term of five years, which would stretch her detention past its supposed expiry date on 27 May and beyond the 2010 elections.
Her lawyers have vowed to contest the charges.
The American man, John Yettaw, was arrested on 6 May after swimming across a lake to her house and staying there secretly for two days. His motives remain unclear.
He will be tried on immigration and security offences, said a lawyer for Ms Suu Kyi. The charges are yet to be confirmed by the government.
The Burmese authorities have described the American as a 53-year-old Vietnam war veteran and resident of the state of Missouri.
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