At 11:10 am Singapore time (0310 GMT), front-month ICE Brent September futures jumped 89 cents/b (1.42%) from Friday's close to $63.23/b, while the NYMEX August light sweet crude futures contract similarly gained 33 cents/b (0.59%) at $56.04/b.The UK-flagged Stena Impero chemicals tanker was attacked Friday by "unidentified small crafts and a helicopter" around 1500 GMT, while transiting the Strait of Hormuz and was heading toward Iran, owner Stena Bulk confirmed.

Iran's Islamic Revolutionary Guard Corps said it seized the Stena Impero "due to a violation of international laws and regulations, while passing through the Strait of Hormuz," according to state TV. It took the tanker to the country's Ports and Maritime Organization for "legal procedures and necessary examinations."

Meanwhile, the UK government asked ships with UK interests to "stay out of the area for an interim period," a government spokeswoman said late Friday.

"We remain deeply concerned about Iran's unacceptable actions which represent a clear challenge to international freedom of navigation," she said after a meeting of ministers and security officials, adding that they will remain in close contact with international partners and hold additional meetings over the weekend.

In a separate incident, the Liberian-flagged VLCC Mesdar was boarded by armed personnel around 1630 GMT and was out of contact, while it was headed north towards Iran, owner Norbulk Shipping said in a statement. By 2000 GMT, the company re-established communication and confirmed that the armed guards had left the vessel and allowed it to continue its voyage.

"While there has been no actual disruption of supply, the market is clearly starting to worry about what further escalation in tension could do in one of the most important trade routes in the oil market," ANZ Research analyst Catherine Birch said in a note.

Supply pressures were further amplified by production disruptions in Libya's Sharara field. The state-owned National Oil Corporation declared force majeure on Sharara crude loaded at Zawiya port due to an" unlawful" closure of a valve.

Analysts noted these factors continue to overshadow looming concerns on demand growth amid ongoing US-China trade tensions.

"Despite the poor demand outlook, we remain neutral-bullish oil prices given that supplies remained tight and geopolitical tensions remain high. We maintain our forecast of Brent at $68/bbl in H2," analysts from OCBC Bank said in a note.

As of 0310 GMT, the US Dollar Index was largely stable, inching up 0.03% at 96.865.