In morning trade, New York's main contract West Texas Intermediate light sweet crude for February delivery advanced 47 cents to $100.12 a barrel.
Brent North Sea crude for February was seven cents higher at $108.08 a barrel.
A showdown between Iran and the United States over Tehran's threats to close the strategic Strait of Hormuz to oil tankers is the main factor influencing short-term crude prices, analysts said.
The United States said Thursday that Iran had exhibited "irrational behavior" by threatening to close a major oil shipping lane it also needs.
"We've seen quite a bit of irrational behaviour from Iran recently," State Department spokeswoman Victoria Nuland said when asked about Tehran's threat to shut down the Strait of Hormuz, a critical passage for more than a third of the world's tanker-borne oil.
The United States and the 27-nation European Union are considering new sanctions aimed at Iran's oil and financial sectors. But EU governments have been divided over whether to impose an embargo on Iranian crude.
Iranian Vice President Mohammad Reza Rahimi has warned that "not a drop of oil will pass through the Strait of Hormuz" if the West adopts sanctions on Iran's oil exports in a bid to curb its nuclear ambitions.
The closure could cause havoc on world oil markets, disrupting the fragile global economy, although analysts say the Islamic republic is unlikely to take such drastic steps as it relies on the route for its own oil exports.
