Britain's ambassador to the UN, John Sawers, who is head of the Security Council this month, said some countries, including China and Russia, had asked for more time to consider a draft statement condemning the verdict.
Ms Suu Kyi was on trial for allowing a US national, John Yettaw, into her lakeside home after he swam there uninvited. Mr Yettaw was jailed for seven years, including four years of hard labour.
Critics of Burma's military regime say the verdict is designed to prevent Ms Suu Kyi from taking part in elections scheduled for 2010.
UN Secretary General Ban Ki-moon has said he "strongly deplores" the verdict and called for Ms Suu Kyi to be freed.
"Unless she and all other political prisoners in Myanmar [Burma] are released and allowed to participate in free and fair elections, the credibility of the political process will remain in doubt," he said.
The UN special envoy to Burma, Ibrahim Gambari, said Ms Suu Kyi was "absolutely indispensable to the resumption of a political process that can lead to national reconciliation".
US President Barack Obama called for her "immediate unconditional release", describing the extension of house arrest as unjust.
Tin Htar Swe, BBC Burmese Service editor
This verdict was unexpected. Aung San Suu Kyi herself was expecting a more severe sentence when she recently told visiting diplomats that her punishment "was obviously going to be painful".
It seems that the ruling party's real intention is to make sure she cannot influence the forthcoming elections in any way.
No one will have access to her without the authorities' approval.
A spokesman for the European Union, Ton van Lierop, said the further detention of the 64-year-old was unacceptable.
"Keeping Aung San Suu Kyi under arrest under fabricated reasons violates her fundamental freedoms, and does not serve the proclaimed national interest either," he told the BBC.
UK Prime Minister Gordon Brown said he was "saddened and angry" by the verdict in what he called a "sham" trial.
In a strongly-worded statement, he condemned the "purely political sentence".
A statement from the office of Nicolas Sarkozy said the French president was calling on the European Union to impose new sanctions on Burma.
Human rights organisations and political parties have been swift to criticise the sentence
Ms Suu Kyi's previous period of house arrest expired on 27 May. This new term will mean she is still in detention during the polls, which are expected to take place in about May 2010.
Her party, the National League for Democracy, won the last elections in 1990 but was never allowed to take power.
In Asia, the governments of Indonesia and the Philippines have been outspoken in condemning the sentence.
But, says the BBC's Jill McGivering, it is notable that two of Burma's biggest trading partners and allies - India and China - have avoided public comment on the trial.
India and China, with Thailand, have been accused by critics of propping up the military government, especially in recent years as growing economic sanctions have strangled its trade relationship with the West.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.