Former US vice president Dick Cheney, who has a long history of heart problems, was hospitalised on Monday after experiencing chest pains, his office said.
The controversial Republican figure, who has been on the offensive against President Barack Obama's administration for months, was "resting comfortably" at George Washington University Hospital in the US capital and his doctors were evaluating the situation, his office said in a statement.
Hospital doctors told NBC News that the former vice president was stable and may receive additional treatment on Tuesday.
The television network said Cheney, 69, apparently received an angiogram test so that doctors could look into his coronary arteries, and that the results showed he may need more treatment.
"We don't know what the plan is," one physician told NBC News.
As recently as February 14, the hawkish architect of former president George W. Bush's "war on terror" said he still supported waterboarding and suggested it could have been used on Nigerian jet bomb suspect Umar Farouk Abdulmutallab.
And in a surprise appearance at a meeting of the Conservative Political Action Conference last Thursday, Cheney boldly declared that Obama would be a "one-term president."
He also predicted that "2010 is going to be a great year for Republicans," with Democrats bracing for losses during crucial mid-term elections in November.
Cheney, who has had four heart attacks since 1978 and had a pacemaker installed in his chest in 2001, was scheduled to meet with former president George W. Bush and other staffers from the Bush administration in Washington on Friday for the first time since they left office in January 2009.
During his third heart attack, Cheney underwent quadruple bypass surgery. His fourth and most recent came in 2000, when he and Bush were elected to the White House.
The battery for Cheney's pacemaker was replaced in 2007 and the entire device was later replaced. He has undergone two artery-clearing angioplasties and been treated twice with electrical shocks for atrial fibrillation, an abnormal heart rhythm that places him at risk of a stroke if not treated.
In 2005, he underwent surgery for an arterial aneurysm on the back of each of his knees.
But despite his persistent health problems, Cheney, who has served four US presidents, emerged as one of the country's most powerful vice presidents and a champion of the Republican right wing.
A principal architect of the war in Iraq and a darling of hardline conservatives, he wielded behind-the-scenes influence in the Bush administration.
Despite his health woes being consistently thrown into the spotlight, Cheney kept pushing on with his duties when in office and has since attacked Obama and his fellow Democrats for being alarmingly naive on national security.
He has also lashed out at his successor, Vice President Joe Biden, calling him "dead wrong" for declaring that another attack on the scale of September 11, 2001 was "unlikely."