Oil prices hit their highest levels since July 2015 on Monday afternoon as markets tightened, while Saudi Arabia's Crown Prince cemented his power over the weekend through an anti-corruption crackdown that included high-profile arrests.

Brent futures, the international benchmark for oil prices, were as high as US$62.90 per barrel, their highest level since July 2015. This is over 40 per cent above June's 2017 lows. US West Texas Intermediate (WTI) crude rose above US$56 per barrel in afternoon trading, also the highest since July 2015, and was at US$56.15 in the afternoon. WTI is one-third higher than its 2017 lows.

Crown Prince Mohammed bin Salman had royals, ministers and investors arrested in the purge.

RBC Capital Markets said in a note that although the "purge represents a stunning political development in Saudi Arabia", it expected "no immediate changes" in the oil policy of Saudi Arabia, the world's biggest exporter of crude oil.

The Prince reforms include a plan to list parts of state-owned oil giant Saudi Aramco next year, and a higher oil price is seen as beneficial for the market capitalisation of the future listed company.

In oil fundamentals, traders said there were ongoing signs of tightening market conditions.

US energy companies cut eight oil rigs last week, to 729, in the biggest reduction since May last year. The decline comes as the Organisation of the Petroleum Exporting Countries (Opec) and a non-Opec group led by Russia pledged to hold back about 1.8 million barrels per day in production to tighten markets.

The pact runs to March next year, but there is growing consensus to extend the deal. While supplies are tightening, analysts say demand remains strong.

REUTERS