"I think it's a big country. It has nuclear weapons that are able to be deployed. It has a real significant minority of radicalized population," Biden said in an interview with CNN.
"It is not a completely functional democracy in the sense we think about it, and so that's my greatest concern."
President Barack Obama's administration has called on Pakistan to see greater urgency in the fight against extremism as the United States pours thousands more troops into Afghanistan to fight Al-Qaeda and Taliban extremists.
US officials have long been concerned that elements in the Pakistani establishment support extremists, despite the nation's offensives against Taliban strongholds in border areas.
They have urged Pakistan to expand its offensive against militants to North Waziristan, a stronghold for Al-Qaeda and the Haqqani network, known for attacking US and NATO troops in Afghanistan.
But Pakistan has chosen not to target so-called Afghan Taliban or some other groups so far and analysts say Islamabad has retained ties to some Islamist militants as a hedge to protect its influence in neighboring Afghanistan.
Earlier this month US Director of National Intelligence Dennis Blair told a senate committee that "vulnerabilities exist" in Pakistan's nuclear safeguards, without elaborating.
The next day he rowed back, trying to be reassuring by saying that the Pakistani military knew there were would be "catastrophic consequences, primarily for Pakistan," if any of its nuclear bombs fell into the wrong hands.