Though the likelihood of any energy supply disruption is remote, Saudi Oil Minister Ali al-Naimi said the world's top oil exporter was willing to supply more crude in the event of a shortage.
Pro-Moscow rebels claimed a resounding victory in Sunday's referendum in eastern Ukraine, with some saying that meant independence and others eventual union with Russia, as clashes broke out between separatists and troops.
Brent crude was up 47 cents at $108.36 by 0836 GMT after touching a 1-1/2-week high of $109.02 on Friday. U.S. crude gained 18 cents to $100.17 after peaking at $101.18 in the prior session, also a 1-1/2-week top.
The European Union, which called Sunday's polling illegal, could tighten sanctions it placed on Russia after Moscow annexed Ukraine's Crimea region following a similar vote in March. But analysts say further action is unlikely to impact supply.
"The EU might come up with more sanctions against Russia, but in terms of supply and demand - that is not going to change anything," said Olivier Jakob of Petromatrix in Zug.
Foreign ministers from the 28-nation bloc are due to meet on Monday to decide whether to add about 15 people and several Crimean-based companies to its list of 48 Russians and Ukrainians already targeted with asset freezes and visa bans.
Investors are also keeping an eye on industrial production and retail sales data expected from China on Tuesday.
Negotiators from Iran and the International Atomic Energy Agency will meet in the Austrian capital Vienna on Monday, a day before talks resume between Tehran and six western nations over Iran's nuclear programme.
The two sets of talks are separate but closely linked as both focus on fears that Iran may be covertly seeking the capability to develop nuclear weapons.
Any sign of progress towards a final resolution to the West's dispute with Tehran would pressure oil prices in anticipation of a ramp-up in Iranian oil exports.
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