"We are monitoring the situation as it unfolds in Iran, but we, like the rest of the world, are waiting and watching to see what the Iranian people decide," Clinton said in Ontario, Canada.
The White House also released a two-sentence statement praising "the vigorous debate and enthusiasm that this election generated, particularly among young Iranians," but also expressing concern over "reports of irregularities."
Joe Biden, the US vice president, said on Sunday that the US would continue with its policy of reaching out to Tehran, with the aim of preventing it from creating nuclear weapons, to which Iran has not yet responded.
Biden said that not enough information was known to say whether the election was fair, but added: "You know I have doubts."
Lawrence Cannon, Canada's foreign affairs minister, said his country was also "deeply concerned" by reports of irregularities in the election.
Frank-Walter Steinmeier, the German foreign minister, said the "course of the election in Iran raises many questions".
He said he had summoned the Iranian ambassador to explain the events following the election.
"I have today, with some European colleagues, already told Iran to immediately make clear whether the election results announced can be taken seriously or not," he said.
In the Netherlands, about 200 demonstrators protested against the vote outside the Iranian embassy in The Hague.
Maxime Verhagen, the Dutch foreign minister, said in a statement that "it seems certain that measures were taken to influence the ballot, such as limiting access to the Internet and blocking the sending of text messages".
In the UK, several hundred Iranians demonstrated in front of the Iranian consulate in London on Saturday, chanting pro-Mousavi slogans and holding up signs that read, "where is my vote?"
"The result is truth-less, it's baseless. Nobody around Iran can say that this result is okay or acceptable for us," one demonstrator said.
David Miliband, Britain's foreign secretary, said he expected the Iranian authorities to address allegations of vote fraud.
"Our priority is that Iran engages with the concerns of the world community, above all on the issue of nuclear proliferation," he said.
The European Union also expressed concern about the alleged fraud, but said it intended to resume dialogue with Tehran over its disputed nuclear programme.
In a statement, the Czech EU presidency noted the re-election of Ahmadinejad for a second four-year term, but also said it was concerned about the violence that has gripped Tehran following the announcement of the official results.
"The presidency hopes that the outcome of the presidential elections will bring the opportunity to resume dialogue on the nuclear issue and clear up the Iranian position in this regard," the EU statement said.
France said that it was monitoring the situation closely.
Bernard Kouchner, France's foreign minister, criticised the Iranian security forces' "somewhat brutal crackdown" on protesters which he said "could leave lasting scars".
Israel has for years expressed concern about Iran, warning it could be building a nuclear weapon.
Danny Ayalon, Israel's deputy foreign minister, said: "The re-election of Ahmadinejad demonstrates the increasing Iranian threat."
Amr Moussa, the Arab League chief, said he hoped Ahmadinejad's second term would boost co-operation to achieve peace and rid the region of weapons of mass
"I believe the situation could move in the direction of quieter talks and understanding. Dialogue is the name of the game," he said.
Iraq's government said it hoped the Iranian leader would seek reconciliation with other countries to promote peace in the region.
Iraq's Shia-led government faces a delicate balancing act in maintaining close ties to both the US and Iran.
Meanwhile, Afghanistan and Pakistan, two important US allies and both neighbours of Iran, made official statements of congratulation to Ahmadinejad.