"The United States is deeply troubled by the conviction today of Egyptian politician Ayman Nur by an Egyptian court," the White House said in a statement on Saturday.
"The conviction of Mr Nur, the runner-up in Egypt's 2005 presidential elections, calls into question Egypt's commitment to democracy, freedom, and the rule of law.
"We are also disturbed by reports that Mr Nur's health has seriously declined due to the hunger strike on which he has embarked in protest of the conditions of his trial and detention.
"The United States calls upon the Egyptian government to act under the laws of Egypt in the spirit of its professed desire for increased political openness and dialogue within Egyptian society, and out of humanitarian concern, to release Mr Nur from detention."
Nur had been detained on 5 December, even before the verdict was reached, and days later began a hunger strike that landed him in a prison hospital last week.
Sean McCormack, a spokesman for the US state department, said: "Mr Nur's trial has been marred by irregularities and inconsistencies and has failed to meet the international standards of transparency and the respect for rule of law that the Egyptian government has publicly espoused."
A criminal court in Cairo jailed the former presidential candidate to five years on Saturday for allegedly forging affidavits to register his Ghad (Tomorrow) political party last year.
The liberties committee of the Egyptian Bar Association decried the ruling, while calls have been made from Egyptian human rights organisations for the attorney-general to reconsider the ruling in view of Nur's ill health.
Talking to Aljazeera, Saad al-Din Ibrahim, a political activist from the Ibn Khaldun Institute, Cairo, said the legitimacy of the judiciary was in question.
"I am in doubt about the integrity of the court that sentenced Nur,” he said.
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"In Egypt we have justice courts branded by the judges and the lawyers as 'private property courts', meaning they give rulings as desired by the government.
"The court that sentenced Nur is one of two such courts well known among the judiciary and the lawyers in Egypt.
"Thus, whenever the ruling regime wanted to implicate any popular figure, they tend to fabricate a case and refer it to one of these two courts."
Ibrahim also highlighted the fact Nur may pose a political challenge to Hosni Mubarak's son Gamal.
"What matters was not his competition to Hosni Mubarak, but to Gamal Mubarak because Nur is as young as Gamal at nearly the same age.
"Nur enjoys a great deal of popularity and has more capabilities in communicating with the masses, while Gamal has no similar capabilities. This is why he poses as a potential and strong rival, which is contrary to the ruling regime's plans to turn to hereditary rule."
Six of Nur's co-defendants received prison terms ranging from three to 10 years.
Nur has always denied the charges and alleged that they were made up by the government to undermine his political career.